Feature to add non-existing users to the turns

[MarckFp]

This is quite useful if someone is visiting the congregation and we want to add turns for them but we don't want them to aggregate them to the pool of the congregation.

A workaround is create them, remove any availability and add them manually but then we have to remove them again if we don't want them to exist in our database

[MarckFp]

Maybe something we can do is to create a new field to the assignment table called tmp_name or something similar. Now if the user_id = -1 it means the user is deleted, but we could add a -2 to the user_id in the assignments table and the front should display the tmp_name instead of the firstname + lastname or the lastname + firstname