Event register

[AlexMog]

Add the Event register system. Task list:

• Add the /me/events endpoint : DONE
• Add the /event/{EVENT_ID}/register endpoint : DONE (POST = register to event, PUT = email validation, DELETE = unregister from event)
• Add the user's email validation endpoint for the register to event process: DONE

[AlexMog]

Need to check that the event is on "Open" mode to allow unregister.