Utility functions for finding lessons

We need utility functions to find lessons by some rules:

• Previous or next lesson of same teacher
• Previous or next lesson of same group(s)
• Previous or next lesson in same subject in this group
• Lessons of a person (student)

changed milestone to %1.0a2

added part::backend prio::1 labels

same: Methode auf dem LessonPeriod-Objekt

3. ⇒ diese Klasse in diesem Fach

1. geht nicht, weil eine Stunde mehrere Lehrer hat

mentioned in commit 6341d76d

marked the checklist item Previous or next lesson of same group(s) as completed

mentioned in commit 6676ef2d

marked the checklist item Previous or next lesson in same subject in this group as completed

Teacher resolution:

1. Check if same constellation exists (similar to 2/3), if so, return that
2. If one of the teachers is the logged-in user, use that
3. Return None otherwise.

1/2: Stundenplanwechsel fehlt noch

marked the checklist item Previous or next lesson of same group(s) as incomplete

marked the checklist item Previous or next lesson in same subject in this group as incomplete

mentioned in commit b11115aa

mentioned in commit 81195d3f

mentioned in commit baa6a974

mentioned in commit da3f326a

mentioned in commit 135db922

mentioned in commit d06b7314