Added bookmarks

• Added:
- bookmarks for GabPRO members only
- migration for creation of StatusBookmarks
- all necessary routes, controllers
- redux for adding, removing, fetching and displaying bookmarks
- bookmark icon
- doorkeeper scopes
- backend and frontend support

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

https://github.com/tootsuite/mastodon/commit/dfea7368c934f600bd0b6b93b4a6c008a4e265b0

db/migrate/20200722190627_create_status_bookmark.rb

@@ -0,0 +1,11 @@
+class CreateStatusBookmark < ActiveRecord::Migration[5.2]
+  def change
+    create_table :status_bookmarks do |t|
+      t.timestamps
+      t.belongs_to :account, foreign_key: { on_delete: :cascade }, null: false
+      t.belongs_to :status, foreign_key: { on_delete: :cascade }, null: false
+    end
+
+    add_index :status_bookmarks, [:account_id, :status_id], unique: true
+  end
+end