[#2115] Allow unsetting judge on a given CASA case

[rhian-cs]

What github issue is this PR for, if any?

Resolves #2115

What changed, and why?

In the edit page of a given CASA case, supervisors and admins can now select a 'blank' judge from the list.

Before that, they couldn't unassign the judge from the case.

How will this affect user permissions?

Permissions have not been affected.

How is this tested? (please write tests!) 💖💪

I added some system specs to check if it was possible to assign, change and unassign a judge from a given case.

Manual testing:

• Go to the edit page of any CASA case
• Check if it is possible to assign a judge and click on Update
• Go to the edit page again
• Check if it the judge dropdown menu now shows a "-Select Judge-" option
• Unassign the judge, and check if the judge is truly unassigned

Screenshots please :)

The "-Select Judge-" option is now available even after assigning a judge to the case.

[compwron]

Very nice! And yay tests :)