Feature flags: add subreddit filtering

Just like turning on a feature for only certain users can be useful, so can
turning it on for only certain subreddits.  In particular, this allows us to
get a little bit of the functionality of a full A/B suite without doing
terribly much work.
Feature: make user check case-insensitive

Since we're doing it for subreddits, we might as well for users, too.  It's an
easy mistake to make, so let's prevent it if we can.