Add GDPR notice and allow data removal

The discussion of General Data Protection Regulation (GDPR) compliance has come up in Discord a few times in recent months. I am submitting this issue so the idea is not forgotten. A boilerplate privacy notice similar to the one employed by phpBB/forum.segs.io can be used for the web UI to notify clients about how SEGS uses their username, IP address, cookie/session data, etc. The web form should also permit a user to remove their stored data if desired after they successfully authenticate.