WIP: Optionally store user IP addresses and display them in AdminAPI, fixes #1708

I know this is controversial just by looking at #1708, but I think it's essential for site moderation purposes. To not have it is to essentially cripple the site moderators, especially on high-traffic sites that deal with a lot of attacks.

The StoreUserIpPlug is optional and disabled by default. It only stores the last known IP of the user. When a user is deleted, the IP is purged from the database. Since it's off by default, issues with GDPR don't really matter, but FWIW Mastodon does this with no option to disable it.