Add duration parameter to muting. (#13831)

* Adding duration to muting.

* Remove useless checks

app/serializers/rest/muted_account_serializer.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class REST::MutedAccountSerializer < REST::AccountSerializer
+  attribute :mute_expires_at
+
+  def mute_expires_at
+    mute = current_user.account.mute_relationships.find_by(target_account_id: object.id)
+    mute && !mute.expired? ? mute.expires_at : nil
+  end
+end