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/privacy/show.html.haml

@@ -4,6 +4,6 @@
 .grid
   .column-0
     .box-widget
-      .rich-formatting= @instance_presenter.site_terms.html_safe.presence || t('terms.body_html')
+      .rich-formatting= @instance_presenter.privacy_policy.html_safe.presence || t('terms.body_html')
   .column-1
     = render 'application/sidebar'