Save initial signup information for users to aid in spam prevention

[techknowlogick]

This will allow instance admins to view signup pattern patterns for public instances. It is modelled after discourse, mastodon, and MediaWiki's approaches.

Note: This has privacy implications, but as the above-stated open-source projects take this approach, especially MediaWiki, which I have no doubt looked into this thoroughly, it is likely okay for us, too. However, I would be appreciative of any feedback on how this could be improved.