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/views/about/show.html.haml

@@ -53,11 +53,11 @@
 
       .hero-widget
         .hero-widget__img
-          = image_tag @instance_presenter.hero&.file&.url || @instance_presenter.thumbnail&.file&.url || asset_pack_path('media/images/preview.png'), alt: @instance_presenter.site_title
+          = image_tag @instance_presenter.hero&.file&.url || @instance_presenter.thumbnail&.file&.url || asset_pack_path('media/images/preview.png'), alt: @instance_presenter.title
 
         .hero-widget__text
           %p
-            = @instance_presenter.site_short_description.html_safe.presence || t('about.about_mastodon_html')
+            = @instance_presenter.description.html_safe.presence || t('about.about_mastodon_html')
             = link_to about_more_path do
               = t('about.learn_more')
               = fa_icon 'angle-double-right'
@@ -66,7 +66,7 @@
           .hero-widget__footer__column
             %h4= t 'about.administered_by'
 
-            = account_link_to @instance_presenter.contact_account
+            = account_link_to @instance_presenter.contact.account
 
           .hero-widget__footer__column
             %h4= t 'about.server_stats'