Add hCaptcha support (#25019)

app/javascript/styles/mastodon/forms.scss

@@ -136,6 +136,10 @@ code {
     line-height: 22px;
     color: $secondary-text-color;
     margin-bottom: 30px;
+
+    a {
+      color: $highlight-text-color;
+    }
   }
 
   .rules-list {
@@ -1039,6 +1043,10 @@ code {
   }
 }
 
+.simple_form .h-captcha {
+  text-align: center;
+}
+
 .permissions-list {
   &__item {
     padding: 15px;