Exam calendar

https://vik.hk has .ics calendars for exams. We should import these, and allow users to enable the ones that are relevant to them. This would make the exams appear in the user's custom .ics URL.

The user should be able to set custom reminder times, which would apply to every event.

## Issue: irrelevant and missing exams
One issue with this, is that especially for later semesters, what subjects a student is studying will probably not match the recommended subjects for the semester, thereby necessitating further customisation.

We could automate this by retrieving subjects the student is currently studyig, and only show exams for those. This creates two problems:

### Retrieving subjects
1. AuthSCH can return these subjects, but this requires the `bme.hu:niifPersonAttendedCourse` scope, which is not available by default.
2. We could extract the subjects from the user's Neptun .ics URL.

### Matching exam times to subjects
The vik.hk calendars use unpredicable abbreviated names for the subjects, which would require manual mapping.

The exam times can also be found at https://vik.bme.hu/page/1150/, but in .pdf form.

We could either ask HK or VIK for something machine-readable, that includes the subject ID and a user-friendly name or parse the PDFs (?).

Start by comparing the vik.hk .ics calendars, the vik.bme.hu exam PDFs, the Neptun .ics URL, and the AuthSCH subject scope described in the issue. Done should cover importing exams, enabling relevant calendars, applying custom reminder times, and producing them in the user's custom .ics URL, with a decided approach for matching subjects.

Written by the indexing model from the issue text.