Task 3125: mentions notifications (NOT TO BE MERGED)

@tanius @albertocottica here you can find a first draft of the implementation for mentions notifications.

The implementation is actually based on:
1) Message
2) Message notify

Entity structure:
- Subject
- Body
- Node/Comment ref.
- Users ref <list>. (mentioned users)

Each notification is saved as a message entity, this means that is virtually possible to build views on top of it (for example a page with nodes where i have been mentioned).

TODO:
1) Comments
2) Something to power-off mentions notifications