keep track of last time an account was used in Balrog

[bhearsum]

While talking about ways to make account rotation easier, Catlee suggested that we should keep track of the last time an account is used in Balrog, which would avoid the need to grovel through logs like we're doing now.

I originally thought we might be able to query this from the existing history tables, but I've since realized that we'd want to include GETs here as well, so we'll probably something different for this. Maybe just a table that is updated whenever a request is made to the admin interface, and updates the timestamp for the user?

We'd also need an API + some UI for it.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1372250)