Allow Join field to also add existing records

[ignissak]

The current implementation of Join field does not allow admin panel users to add existing records to the relationship, they can only edit current connected records or create a brand new record.

For example:

• a student is assigned to one class (in collection this is a relationship field)
• class can have multiple students (in collection this is a join field)
• I want to use the relationship table UI because it is more convenient than the UI using in relationship fields
• admin panel users can see the students assigned in the class edit view but cannot assign more students, they have to go manually to every student and set the class there