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


I think we might want proper validation, I don't know much about it since I haven't written the code. ping @aenario if you have a quick idea of the right way of doing thing or at least a better understanding than I do.