This is the first step to include a visual calendar to Poulpe,
only including a "day view" as of now (i.e. events hour-by-hour from one date to another).
More views may be added at a later time.
It is **NOT WORKING YET**!
The CSS has been broken on this branch, and will have to be fixed before the calendar can work:
* the CSS of the calendar needs to be adapted to the environment and design of Poulpe;
* add actual links to enroll/un-enroll to an activity (cf. `Event` class in `calendar.js`);
* other small tweaks :)?
Finally, this view is likely to require the addition of start and end date change,
so that an user can browse events over several days (cf. `setStartDate` and `setEndDate` methods of `Calendar` class in `calendar.js`).
Note that this code should be better re-written (e.g. in Typescript, split between files, using more/better design patterns) at a later time.
It should nonetheless be easy to fix it and use it right now (see above requirements for this).
