activity : add several periods

[remipichon]

No description provided.

[LeoMouyna]

@remipichon
Looks like Activity collection is set to only accept one time slot.
I suggest to refactor this with a time slot class and a time slot array for activities, what do you think ?
This time slot class could be useful for other collections too. Let me know.

[remipichon]

Timeslot is too complet (it has a lot of stuff for people needs), here you use an
array of start/end date (via another small schema).

On the ui, loop though those dates and allow for a "add". You can use the autoField to autoForm like 'description' to first play with it, then add a custom ui.