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/controllers/about_controller.rb

@@ -20,7 +20,7 @@ class AboutController < ApplicationController
   def more
     flash.now[:notice] = I18n.t('about.instance_actor_flash') if params[:instance_actor]
 
-    toc_generator = TOCGenerator.new(@instance_presenter.site_extended_description)
+    toc_generator = TOCGenerator.new(@instance_presenter.extended_description)
 
     @rules             = Rule.ordered
     @contents          = toc_generator.html