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.

config/routes.rb

@@ -639,10 +639,12 @@ Rails.application.routes.draw do
     end
 
     namespace :v2 do
-      resources :media, only: [:create]
       get '/search', to: 'search#index', as: :search
+
+      resources :media,       only: [:create]
       resources :suggestions, only: [:index]
       resources :filters,     only: [:index, :create, :show, :update, :destroy]
+      resource  :instance,    only: [:show]
 
       namespace :admin do
         resources :accounts, only: [:index]