Add teachers to the timetable

[nilsreichardt]

Description

This pull request adds the Sharezone Plus feature to add teachers to the timetable.

Demo

https://github.com/SharezoneApp/sharezone-app/assets/24459435/51576d4d-c5ac-42be-bee9-04084cb228a5

Changed the lesson sheet to a dialog:

Reason: In the sheet you need to scroll to see the substitutions tiles and it's not easy to adjust the size of the sheet (depending on the screen size). A dialog is here the better option.

Users can also add the teacher when adding a lesson:

Sharezone Plus page:

Related Tickets

Closes #1608 Opens #1628

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[nilsreichardt]

Idea: Split teachers at ","