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/api/v2/instances_controller.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class Api::V2::InstancesController < Api::V1::InstancesController
+  def show
+    expires_in 3.minutes, public: true
+    render_with_cache json: InstancePresenter.new, serializer: REST::InstanceSerializer, root: 'instance'
+  end
+end