featreq: event registration time limit should be configurable

[pr0m3th3usEx]

Requested by user

• Event registration time limit should be configurable by the organizer when creating or editing an event
• Registration time limit should be the end of the event by default
• Display message if the event is already started
• Update EventParticipation.TooLate component

[pr0m3th3usEx]

@MikaelVallenet Some backend work is required for this issue, right ?