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/features/ui/components/report_modal.js

@@ -2,7 +2,7 @@ import React from 'react';
 import { connect } from 'react-redux';
 import { submitReport } from 'mastodon/actions/reports';
 import { expandAccountTimeline } from 'mastodon/actions/timelines';
-import { fetchRules } from 'mastodon/actions/rules';
+import { fetchServer } from 'mastodon/actions/server';
 import PropTypes from 'prop-types';
 import ImmutablePropTypes from 'react-immutable-proptypes';
 import { makeGetAccount } from 'mastodon/selectors';
@@ -117,7 +117,7 @@ class ReportModal extends ImmutablePureComponent {
     const { dispatch, accountId } = this.props;
 
     dispatch(expandAccountTimeline(accountId, { withReplies: true }));
-    dispatch(fetchRules());
+    dispatch(fetchServer());
   }
 
   render () {