Ensure at most one approved claim per facility

[kellyi]

Overview

When attempting to approve a facility claim, check whether there are
already any other approved facility claims and, if so, return a bad
request response. Also add a test to demonstrate the feature
functionality.

Connects #581

Testing Instructions

It should suffice just to run the tests and verify that they continue to pass.

You can also go through the loop of:

• turn on claim a facility
• sign in as c2@example.com
• submit the claim form 2x for the same facility
• sign out, then sign in as a superuser
• visit /dashboard/claims
• approve one of the facilities and verify that it works
• attempt to approve the other facility and verify that it does not work and that you see an error

Checklist

• fixup! commits have been squashed
• CI passes after rebase
• CHANGELOG.md updated with summary of features or fixes, following Keep a Changelog guidelines

[rajadain]

Taking a look now.

[rajadain]

Works as advertised. It would've been nicer if the error explained that the reason for failure was the presence of an approve claim, rather than the generic "there was an error", but that can be an improvement down the line.

I was able to successfully deny the second claim, and it did not affect the way the facility is shown in the front-end. It still shows the first claim correctly.

[kellyi]

Thanks for testing this! For this:

It would've been nicer if the error explained that the reason for failure was the presence of an approve claim, rather than the generic "there was an error"

The server error will be logged to the browser console, so we can always get it that way we encounter this again.