Add backend support for bookmarks

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

app/models/bookmark.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: bookmarks
+#
+#  id         :integer          not null, primary key
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#  account_id :integer          not null
+#  status_id  :integer          not null
+#
+
+class Bookmark < ApplicationRecord
+  include Paginable
+
+  update_index('statuses#status', :status) if Chewy.enabled?
+
+  belongs_to :account, inverse_of: :bookmarks
+  belongs_to :status,  inverse_of: :bookmarks
+
+  validates :status_id, uniqueness: { scope: :account_id }
+
+  before_validation do
+    self.status = status.reblog if status&.reblog?
+  end
+end