refactor: shorten registration events

[jovialen]

Shorten registration events

Resolves #3714. Shortens registration events from 30 to 15 minutes to better fit into NTNU calendar.

[jonasdeluna]

Awesome that you want to contribute 👍👍
We will probably discuss this first before potentially merging

[jovialen]

It's just an annoyance of mine. Personally, I'd prefer the registration events to look a bit thin over obstructing my classes in the calendar. I'll cross my fingers that you'll agree :)

[eikhr]

We discussed this and did some testing in our calendars. It seems like the event could be even shorter than 15 mins and still be displayed fine in our calendars. Is there any reason why you went with 15, instead of 5 (or even 0!) minutes?

[jovialen]

I choose 15 minutes simply because it was the smallest reduction which would fit the registration events into the between-lectures slot in the calendar. Further reducing it to 0 minutes would probably be completely fine, but I chose not to do so for two reasons:

1. I wasn't sure it wouldn't cause problems with some calendars
2. Doing so would also warrant refactoring the code to remove the registration completely, and I wanted to avoid making big changes to the codebase uninvited.

If these reasons aren't concerning, I could probably add the aforementioned refactor to this pull request.