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


can you please add some comments in the .reduce function to make it more clear what we want, what we have, etc. (various cases)