Change public accounts pages to mount the web UI (#19319)

* Change public accounts pages to mount the web UI

* Fix handling of remote usernames in routes

- When logged in, serve web app
- When logged out, redirect to permalink
- Fix `app-body` class not being set sometimes due to name conflict

* Fix missing `multiColumn` prop

* Fix failing test

* Use `discoverable` attribute to control indexing directives

* Fix `<ColumnLoading />` not using `multiColumn`

* Add `noindex` to accounts in REST API

* Change noindex directive to not be rendered by default before a route is mounted

* Add loading indicator for detailed status in web UI

* Fix missing indicator appearing while account is loading in web UI

app/javascript/mastodon/features/explore/index.js

@@ -84,6 +84,7 @@ class Explore extends React.PureComponent {
 
               <Helmet>
                 <title>{intl.formatMessage(messages.title)}</title>
+                <meta name='robots' content={isSearching ? 'noindex' : 'all'} />
               </Helmet>
             </React.Fragment>
           )}