Introduce default and firehose channels

Default channels get bugs that haven't been sent anywhere
else. Firehose channels get all bugs, no matter what.

In practice, this means that #wikimedia-dev will not get all the bugs
anymore. Bugs that are reported in other channels will not show up
in -dev. This should reduce bugnoise on -dev a fair bit, and also match
the behavior of grrrit-wm.

#mediawiki-feed will still get the unfiltered firehose of all bug changes.