Modularize the mentions data consumers

Currently, only users of the wiki can be mentioned.

In the future, we want to be able to mention different kind of actors such as groups, or users of remote wikis (using ActivityPub).

To do so, different implementations of the mentions data consumer component must be proposed and used according to the kind of the mentioned actor.

Implications:

• update DefaultMentionsEventExecutor to select the relevant data consumers
• update the MentionMacro to introduce a type parameter, identifying the type of the mention