Add a sanitize-profile command

We get a lot of spam users that use the profile of a user as spam source, setting bio, website, linkedin, etc.

The new command damspam sanitize-profile reset those fields to empty strings. This is intended to be run from the user-create hook but can be run manually from the CLI.

Since the profile is filled out after user creation, a default delay of 10 minutes applies to make sure we check once the profile is actually filled in.

Note that gitlab doesn't support resetting an avatar. Instead, we grab the gravatar image that gitlab uses in its API docs and upload that as the new gravatar.

cc @bentiss