DONOTMERGE - Guillaume works

Brake compatibilities with actual cozy-calendar and actual cozy-sync, to achieve these :
* changed date convention in database: data and API is in UTC, the client has the Cozy's timezone
* client's dates are now handled with moment-timezone
* added all-day events support
* added event's reminder support


Please improve the block comment (like the one in alarm) if relevant (it might stay as-is if you think so) and place it before `Event::toIcal`