Allow null group assignment for users

[lcharette]

This will probably require a db migration to allow nullable value. The UI need to be allow for a "no group" option.

(Ref #843)

[amosfolz]

Since this will require another DB migration I thought it might be good to include it in the same release as #957 since that will require one as well. If there are no problems I will start working on it.

[amosfolz]

I am running into some problems trying to figure out the best way to handle this. I am running into foreign key constraints from the relationships added here https://github.com/userfrosting/UserFrosting/blob/master/app/sprinkles/account/src/Database/Migrations/v420/AddingForeignKeys.php#L44

Even if we make the users.group_id column nullable, we will not be able to save a value unless it also exists in the groups.id

[lcharette]

That's because the group_id is 0 while it should be null.

See : https://stackoverflow.com/a/15082906/445757

[amosfolz]

That's because the group_id is 0 while it should be null.

See : https://stackoverflow.com/a/15082906/445757

Can we use value="0" on the form, then in the controller do a check? Something like this right before the update is made:

       if ($data['group_id'] == 0) {
           $data['group_id'] = null;
       }

Otherwise validation fails if group_id is not a integer.

[amosfolz]

Was addressed in #964.