Fix behavior for Enumerable-like ActiveRecord relations.

app/models/concerns/participable.rb

@@ -32,12 +32,10 @@ module Participable
     case value
     when User
       [value]
-    when Array
+    when Enumerable, ActiveRecord::Relation
       value.flat_map { |v| participants_for(v, current_user) }
     when Participable
       value.participants(current_user)
-    when Mentionable
-      value.mentioned_users(current_user)
     end
   end
 end