Add support for multiple excuse types

At the Katharineum students can be excused for school topics or common topics.

changed milestone to %1.0a1

added part::backend part::frontend prio::1 source::customer labels

Concept draft

• Model ExcuseType with fields name and short_name
• ForeignKey to ExcuseType at personal notes (in addition to excused) [can be null]
• Every excuse type is counted extra, as well as if excuse type is null

@nik What's your opinion?

created merge request !64 (merged) to address this issue

mentioned in merge request !64 (merged)

assigned to @hansegucker

closed via merge request !64 (merged)

mentioned in commit 161d38c4