Add server banner to web app, add GET /api/v2/instance to REST API (#19294)

Cherry-picked d2528b26b6

Conflicts:
- `app/serializers/initial_state_serializer.rb`:
  Upstream changed stuff, we had extra attributes.
  Applied upstream changes while keeping our extra attributes.
- `app/serializers/rest/instance_serializer.rb`:
  Upstream actually moved that to `app/serializers/rest/v1/instance_serializer.rb`,
  so updated that file by keeping our extra attributes, and took upstream's
  version of `app/serializers/rest/instance_serializer.rb`.
- `spec/views/about/show.html.haml_spec.rb`:
  Took upstream's version.

app/javascript/mastodon/components/display_name.js

@@ -2,11 +2,12 @@ import React from 'react';
 import ImmutablePropTypes from 'react-immutable-proptypes';
 import PropTypes from 'prop-types';
 import { autoPlayGif } from 'mastodon/initial_state';
+import Skeleton from 'mastodon/components/skeleton';
 
 export default class DisplayName extends React.PureComponent {
 
   static propTypes = {
-    account: ImmutablePropTypes.map.isRequired,
+    account: ImmutablePropTypes.map,
     others: ImmutablePropTypes.list,
     localDomain: PropTypes.string,
   };
@@ -48,7 +49,7 @@ export default class DisplayName extends React.PureComponent {
       if (others.size - 2 > 0) {
         suffix = `+${others.size - 2}`;
       }
-    } else {
+    } else if ((others && others.size > 0) || this.props.account) {
       if (others && others.size > 0) {
         account = others.first();
       } else {
@@ -63,6 +64,9 @@ export default class DisplayName extends React.PureComponent {
 
       displayName = <bdi><strong className='display-name__html' dangerouslySetInnerHTML={{ __html: account.get('display_name_html') }} /></bdi>;
       suffix      = <span className='display-name__account'>@{acct}</span>;
+    } else {
+      displayName = <bdi><strong className='display-name__html'><Skeleton width='10ch' /></strong></bdi>;
+      suffix = <span className='display-name__account'><Skeleton width='7ch' /></span>;
     }
 
     return (