Add purge-user and purge-spammers

On top of !4 (merged) but just because I'm lazy

This adds two damspam commands: purge-user and purge-spammers. The former shows the given user name, the latter lists all blocked users with at least one issue labelled "Spam". In both cases there's an interactive prompt to confirm deleting the users.

The purge-spammers command is the counterpart for damspam's webhook functionality. Since the hide-issue-hook will block the user, that user can no longer be reported and just doesn't show up in the admin interface. This makes getting rid of the user (and their contributions) a lot more clicky, so having a simple command admins can run every so-often is useful.

edit: a few more features:

• purge-user will refuse to purge an Adminstrator or the spambot itself
• if hide-issue/hide-issue-hook fails for some pre-checks, the spambot will add a comment to the issue so the devs know it didn't do anything.
• hide-issue/hide-issue-hook now also assigns the issue to the spambot - and that is then used as filtering so where we recursively tag commits as spam, ignoring those already assigned to us means we only process those once