Add duration parameter to muting. (#13831)

* Adding duration to muting.

* Remove useless checks

db/migrate/20200317021758_add_expires_at_to_mutes.rb

@@ -0,0 +1,5 @@
+class AddExpiresAtToMutes < ActiveRecord::Migration[5.2]
+  def change
+    add_column :mutes, :expires_at, :datetime
+  end
+end