Add calendar endpoint

[olillin]

This pull request adds GET /cal.ics which generates a calendar file containing the study weeks in a time period around the current date. It also adds a dependency, iamcal, which is used to generate the calendar. The library is maintained by me.

The time period defaults to 8 weeks before and after the current date. The time period can be customized by the before and after query parameters which can be between 0 and a maximum value defined by the environment variables MAX_BEFORE and MAX_AFTER respectively (both default to 20 weeks).

The events are full-day events on every Monday with the study week as the summary (see screenshot below).

[Oscariremma]

Looks good!
One thing we might want to do is to add a link to the ical endpoint from the main page so that it is discoverable (and also update the README).
But if that is not something you feel like adding we can probably add it later.

[olillin]

One thing we might want to do is to add a link to the ical endpoint from the main page so that it is discoverable (and also update the README). But if that is not something you feel like adding we can probably add it later.

Unfortunately, I do not have time to implement this myself at the moment. But if you want to do it yourself you could look at when I did this for nollk.it: cthit/nollk.it#72

[olillin]

I have started working on a panel to subscribe to the calendar now, not quite happy with the design though so I will keep working on it some more.

[olillin]

Looks like this now, bigger text definitely fits better with the large week title.

[olillin]

@Oscariremma I found some spare time and have implemented the requested changes. Let me know if there is anything that needs changing!

I also made the decision to break out the CSS and JavaScript into separate files because there was suddenly a lot more of it and it was easier to develop this way (and likely to maintain)

[Oscariremma]

Looks great and thanks for the contribution!