Add notifications for new sign-ups (#16953)

app/javascript/mastodon/reducers/settings.js

@@ -37,6 +37,7 @@ const initialState = ImmutableMap({
       poll: false,
       status: false,
       update: false,
+      'admin.sign_up': false,
     }),
 
     quickFilter: ImmutableMap({
@@ -57,6 +58,7 @@ const initialState = ImmutableMap({
       poll: true,
       status: true,
       update: true,
+      'admin.sign_up': true,
     }),
 
     sounds: ImmutableMap({
@@ -68,6 +70,7 @@ const initialState = ImmutableMap({
       poll: true,
       status: true,
       update: true,
+      'admin.sign_up': true,
     }),
   }),