User preferred song key #145

Is your feature request related to a problem? Please describe.
Currently, a song can be transposed to any key on the fly and a custom key can be saved for songs on a setlist. When creating a setlist to perform with different people, there is currently no way to tell, if others have a preferred key for a specific song (e.g. because of the voice range of a singer).

Describe the solution you'd like

• Users should have the possibility to define a preferred key for a song and save it to their user account
• Setlists should have an additional field co-performers, where other musicians or singers can be added
• Setlists should show the preferred keys of all co-performers for the creator to decide the modification of the custom song keys of this setlist accordingly

Additional context
Dependend on #201.