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


Cf previous discussion : https://github.com/cozy/cozy-ical/pull/7#discussion-diff-18816588 ; put back the TODO, or add the functionnality ?