Allow admins to purge USER keys on-demand

[eddiewebb]

Expose API to purge key for specific user, or a specific key even though it may not be expired based on time limits. (was compromised, etc)

This should remove the keys and trigger an email to the user to follow up.

[ccunniff]

Is this issue valid? The stash ssh plugin provides an endpoint for this already https://developer.atlassian.com/static/rest/stash/3.11.3/stash-ssh-rest.html#idp171136

Admins can also delete a user's ssh key(s) through the UI /admin/users/view?name=<usename>

It probably doesn't send an email to the user though.