bot_seen: support LIKE queries for Dries, Dries_, DriesAFK, etc.

"seen Dries" should report the entries for LIKE 'Dries%', so that it'd catch Dries_, DriesAFK, and so forth. There's a certain amount of danger in this - one could flood the channel with tons of garbage given enough effort (say, "seen a?"). Something we may want to consider instead is just using the latest value (if Dries_ was the latest timestamp'd match, show that one and potentially just report the other names we know ("see also: Dries, DriesAFK")).