Add backend support for bookmarks

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

db/migrate/20180410220657_create_bookmarks.rb

@@ -0,0 +1,14 @@
+class CreateBookmarks < ActiveRecord::Migration[5.1]
+  def change
+    create_table :bookmarks do |t|
+      t.references :account, null: false
+      t.references :status, null: false
+
+      t.timestamps
+    end
+
+    add_foreign_key :bookmarks, :accounts, column: :account_id, on_delete: :cascade
+    add_foreign_key :bookmarks, :statuses, column: :status_id, on_delete: :cascade
+    add_index :bookmarks, [:account_id, :status_id], unique: true
+  end
+end