iCal export for school vacations

[wintermeyer]

Users want to download iCal https://en.wikipedia.org/wiki/ICalendar exports of our calendars. For federal states and schools. We need to provide the following versions:

• One calendar with just the school vacations (is_school_vacation == true).
• One calendar with the school vacations and the public holidays.

Both will be linked on https://www.mehr-schulferien.de/ferien/d/bundesland/:slug and on https://www.mehr-schulferien.de/ferien/d/schule/:slug

I don't know under which path they should be routed (I guess /api/2.0/...).

Misc

I am not familiar with the iCal format but I guess it contains some sort of fingerprint to make sure if data has changed. We have to honor that system by querying the updated_at fields of the used periods and use that as that fingerprint.

[wintermeyer]

I don't know how good this is but I just found it: https://github.com/lpil/icalendar

[riverrun]

Mostly finished in #532

Remaining tasks:

• add links to the federal_state page
• changed the wording and update the formatting of the links on the school page
• decide what filenames we want to use and if we want different filenames for the different types of icalendars

[wintermeyer]

• decide what filenames we want to use and if we want different filenames for the different types of icalendars

I think taking the name of the location and the year is a good first choice.

[riverrun]

Updated in #533

The wording and formatting of the icalendar links still need to be updated.