This gets rid of some duplicate code and makes it simpler to add
additional support channels in future.

Most slack functions will accept either a channel name or a channel
ID. We always use a channel ID for support channels for the few that
require it. For readability, our settings have used channel names, but the
disadvantage is that if a channel name changes, bennettbot will no
longer be able to post to it.

This allows settings to be either channel name or channel ID. If we
decide to change them to channel IDs in prod, this means channel names
can be changed with no interruption to how the support keywords work.

Both the bot and the message checker in the dispatcher use elements
of the support config, so move it out into a shared module.

I'm not greatly happy about having a config.py that this lives in,
but I don't want to pollute settings.py with functions that build
config, and I don't want dispatcher.py to be dependent on bot.py

Also fixes a bug with the reaction emoji config, and uses
"support_channel" as the key in the support config to be more
explicit about which channel we're referring to in functions that
use this config.