Add teachers to substitution plan

[nilsreichardt]

Demo

https://github.com/SharezoneApp/sharezone-app/assets/24459435/f9b89af0-f914-4ad5-abca-ff6e1d927745

Summary

A user can add substitution teachers. Free users can see the teachers now. Substitutions are a list on the client now.

Description

This PR adds the option to add substitution teachers.

I changed that now free users can also see the teachers. Reason for this: It doesn't really makes sense if a user can see the substitution teacher, but not the normal teacher.

Before this PR, a lesson had only one substitution at the time. Now, a lesson can have multiple substitutions (like room substitution and teacher substitution). This not a breaking change thanks to a robust database design.

Related Tickets

Closes #1628

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[Jonas-Sander]

Very nice! LGTM