Improve scss color variables (#2912)

- Updates scss variables file to use better-named variables for black/white/etc
- Arranges the "mastodon classic" colors into variables
- Remove all references to `$color-*` naming, replacing with new

This does not in itself introduce "theme" support, but:

- It would probably be easier to start working on theme support after this
  change and others
- Even without the goal of themes, these changes make it more clear how the
  colors are being used.

There is almost definitely some edge case in here where I've guessed the
intent/context of some color usage incorrectly, but it still seems like a net
improvement.

app/javascript/styles/forms.scss

@@ -16,7 +16,7 @@ code {
 
   span.hint {
     display: block;
-    color: $color3;
+    color: $ui-primary-color;
     font-size: 12px;
     margin-top: 4px;
   }
@@ -51,7 +51,7 @@ code {
     label {
       font-family: inherit;
       font-size: 16px;
-      color: $color5;
+      color: $primary-text-color;
       display: block;
       padding-top: 5px;
     }
@@ -65,7 +65,7 @@ code {
     margin-bottom: 5px;
     font-family: inherit;
     font-size: 14px;
-    color: $color5;
+    color: $primary-text-color;
     display: block;
     width: auto;
   }
@@ -76,7 +76,7 @@ code {
     label {
       font-family: inherit;
       font-size: 14px;
-      color: $color5;
+      color: $primary-text-color;
       display: block;
       width: auto;
     }
@@ -108,11 +108,11 @@ code {
     background: transparent;
     box-sizing: border-box;
     border: 0;
-    border-bottom: 2px solid $color3;
+    border-bottom: 2px solid $ui-primary-color;
     border-radius: 2px 2px 0 0;
     padding: 7px 4px;
     font-size: 16px;
-    color: $color5;
+    color: $primary-text-color;
     display: block;
     width: 100%;
     outline: 0;
@@ -124,35 +124,35 @@ code {
     }
 
     &:focus:invalid {
-      border-bottom-color: $color6;
+      border-bottom-color: $error-value-color;
     }
 
     &:required:valid {
-      border-bottom-color: $color7;
+      border-bottom-color: $valid-value-color;
     }
 
     &:active,
     &:focus {
-      border-bottom-color: $color4;
-      background: rgba($color8, 0.1);
+      border-bottom-color: $ui-highlight-color;
+      background: rgba($base-overlay-background, 0.1);
     }
   }
 
   .input.field_with_errors {
     label {
-      color: $color6;
+      color: $error-value-color;
     }
 
     input[type=text],
     input[type=email],
     input[type=password] {
-      border-bottom-color: $color6;
+      border-bottom-color: $error-value-color;
     }
 
     .error {
       display: block;
       font-weight: 500;
-      color: $color6;
+      color: $error-value-color;
       margin-top: 4px;
     }
   }
@@ -167,8 +167,8 @@ code {
     width: 100%;
     border: 0;
     border-radius: 4px;
-    background: $color4;
-    color: $color5;
+    background: $ui-highlight-color;
+    color: $primary-text-color;
     font-size: 18px;
     padding: 10px;
     text-transform: uppercase;
@@ -181,26 +181,26 @@ code {
     margin-bottom: 10px;
 
     &:hover {
-      background-color: lighten($color4, 5%);
+      background-color: lighten($ui-highlight-color, 5%);
     }
 
     &:active,
     &:focus {
       position: relative;
       top: 1px;
-      background-color: darken($color4, 5%);
+      background-color: darken($ui-highlight-color, 5%);
     }
 
     &.negative {
-      background: $color6;
+      background: $error-value-color;
 
       &:hover {
-        background-color: lighten($color6, 5%);
+        background-color: lighten($error-value-color, 5%);
       }
 
       &:active,
       &:focus {
-        background-color: darken($color6, 5%);
+        background-color: darken($error-value-color, 5%);
       }
     }
   }
@@ -211,12 +211,12 @@ code {
 }
 
 .flash-message {
-  background: $color1;
-  color: $color3;
+  background: $ui-base-color;
+  color: $ui-primary-color;
   border-radius: 4px;
   padding: 15px 10px;
   margin-bottom: 30px;
-  box-shadow: 0 0 5px rgba($color8, 0.2);
+  box-shadow: 0 0 5px rgba($base-shadow-color, 0.2);
   text-align: center;
 
   strong {
@@ -229,7 +229,7 @@ code {
   text-align: center;
 
   a {
-    color: $color5;
+    color: $primary-text-color;
     text-decoration: none;
 
     &:hover {
@@ -242,7 +242,7 @@ code {
 .follow-prompt {
   margin-bottom: 30px;
   text-align: center;
-  color: $color3;
+  color: $ui-primary-color;
 
   h2 {
     font-size: 16px;
@@ -250,7 +250,7 @@ code {
   }
 
   strong {
-    color: $color2;
+    color: $ui-secondary-color;
     font-weight: 500;
   }
 }
@@ -261,10 +261,10 @@ code {
 
 .qr-code {
   flex: 0 0 auto;
-  background: $color5;
+  background: $simple-background-color;
   padding: 4px;
   margin-bottom: 20px;
-  box-shadow: 0 0 15px rgba($color8, 0.2);
+  box-shadow: 0 0 15px rgba($base-shadow-color, 0.2);
   display: inline-block;
 
   svg {
@@ -275,7 +275,7 @@ code {
 
 .qr-alternative {
   margin-left: 10px;
-  color: $color3;
+  color: $ui-primary-color;
 
   samp {
     display: block;
@@ -296,16 +296,16 @@ code {
   .warning {
     max-width: 400px;
     box-sizing: border-box;
-    background: rgba($color6, 0.5);
-    color: $color5;
-    text-shadow: 1px 1px 0 rgba($color8, 0.3);
-    box-shadow: 0 2px 6px rgba($color8, 0.4);
+    background: rgba($error-value-color, 0.5);
+    color: $primary-text-color;
+    text-shadow: 1px 1px 0 rgba($base-shadow-color, 0.3);
+    box-shadow: 0 2px 6px rgba($base-shadow-color, 0.4);
     border-radius: 4px;
     padding: 10px;
     margin-bottom: 15px;
 
     a {
-      color: $color5;
+      color: $primary-text-color;
       text-decoration: underline;
 
       &:hover,