Add backend support for bookmarks

Bookmarks behave like favourites, except they aren't shared with other
users and do not have an associated counter.

config/routes.rb

@@ -224,6 +224,9 @@ Rails.application.routes.draw do
           resource :favourite, only: :create
           post :unfavourite, to: 'favourites#destroy'
 
+          resource :bookmark, only: :create
+          post :unbookmark, to: 'bookmarks#destroy'
+
           resource :mute, only: :create
           post :unmute, to: 'mutes#destroy'
 
@@ -259,6 +262,7 @@ Rails.application.routes.draw do
         end
       end
       resources :favourites, only: [:index]
+      resources :bookmarks,  only: [:index]
       resources :reports,    only: [:index, :create]
 
       namespace :apps do