Ability to put multiple users in a User Select field

Right now a user select can only have one user, and once a user is selected it isn't possible to remove one.
The api also returns an object instead of an array, limiting the value to 1 user:

Goals:
- a user select that can have 0, 1 or more users
- ability to configure the field to be limited to 1 user
- not breaking the many existing plugins and the mobile app that interact with the assigned_to field

Discussion:
- we could just use the connection field and limit the responses to users. This would leverage the user's contact instead of the user record.
- do we change the existing field type or do we introduce a new field type that returns an array instead of an object?