Add duration parameter to muting. (#13831)

* Adding duration to muting.

* Remove useless checks

app/models/mute.rb

@@ -9,11 +9,13 @@
 #  account_id         :bigint(8)        not null
 #  target_account_id  :bigint(8)        not null
 #  hide_notifications :boolean          default(TRUE), not null
+#  expires_at         :datetime
 #
 
 class Mute < ApplicationRecord
   include Paginable
   include RelationshipCacheable
+  include Expireable
 
   belongs_to :account
   belongs_to :target_account, class_name: 'Account'