Change followers page to relationships page in admin UI (#12927)

Allow browsing and filtering all relationships instead of just
followers, unify the codebase with the user-facing relationship
manager, add ability to see who the user invited

config/routes.rb

@@ -223,7 +223,7 @@ Rails.application.routes.draw do
       resource :reset, only: [:create]
       resource :action, only: [:new, :create], controller: 'account_actions'
       resources :statuses, only: [:index, :show, :create, :update, :destroy]
-      resources :followers, only: [:index]
+      resources :relationships, only: [:index]
 
       resource :confirmation, only: [:create] do
         collection do