Allow users to be deleted

[zwass]

Currently, users can be disabled (which logs them out and prevents them from logging back in), but cannot be deleted. It makes sense to add an action to permanently delete users.

[nyanshak]

:wave: I'm starting a branch to work on this. Hopefully will be able to get something up in the next day or so, but may need help with fleetctl / frontend integration of the changes.

[nyanshak]

Made a PR to add the backend endpoints. I realized that fleetctl doesn't have a lot of facilities to work with users in general, so figured that was a bigger piece of work to separate out if needed.

I'm not very familiar with the frontend code yet, so I'll stick to adding the API endpoints in the PR linked.