(cherry picked from commit 7645d34d)

(cherry picked from commit e4060943)

(cherry picked from commit bfcc6f33)

(cherry picked from commit 68b2c778)

(cherry picked from commit 53e2eabd)

This can be reverted completely when working on #7 outside of the release cycle.

Out of validity = Linked to a validity, but the validity range ends before the substitution's date

Close #15

Close #16

Currently translated at 100.0% (38 of 38 strings)

Translation: AlekSIS/AlekSIS-App-Untis
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-untis/de/

Currently translated at 2.2% (1 of 44 strings)

Translation: AlekSIS/AlekSIS-App-Untis
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis-app-untis/la/

Close #8

Reason: UNTIS school years are always one full year
and school terms also can be a half year.