The popup is horizontally aligned with the click location, and
vertically close to event (slightly on top of it, from below).
If the click occurs too much on the left or the right side of the
screen, the horizontal position is shifted accordingly, so that it is
never displayed outside of the screen. Note, however, that the small
arrow supposed to point the related event is not shifted as well as of
now (not as straightforward since it is a CSS pseudo-element).
This commit also removes the ability to display the popup _above_ the
event in case the click occurs too close to the bottom of the screen.
This feature should be reintroduced by a later commit once it will have
been fixed :).
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).
