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/application/_sidebar.html.haml

@@ -1,9 +1,9 @@
 .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')
+    %p= @instance_presenter.description.html_safe.presence || t('about.about_mastodon_html')
 
 - if Setting.trends && !(user_signed_in? && !current_user.setting_trends)
   - trends = Trends.tags.query.allowed.limit(3)