MentionsInterface now also extends the EntityOwnerInterface

The Mentions entity already implemented some of the EntityOwnerInterface methods and places using this entity also relied on those methods being present. This change ensures that things that say they are a compatible with usages of the Mentions entity, actually are. It now also implements the setOwnerId and setOwner methods as convenience methods in order to adhere to the Interface.

Because all users of a Mentions entity can work with any class that implements the MentionsInterface, this interface was adjusted instead of modifying the Mentions entity's interface declarations.