Add --watched-channel flag, to allow-list specific channels

[conorsch]

Galileo currently listens to chatter in all Discord channels it has access to, and responds by default. This means that addresses can get posted in a lot of different channels, and our Discord admins have to explicitly ban Galileo from responding in those channels. That's no good. Instead, let's add a --watched-channel flag, which can be repeated, then we'll narrowly target only specific channels that Galileo will listen for messages on.

[conorsch]

With some judicious Discord role munging, I believe we have Galileo restricted to the appropriate channels now. However, debug logs are generated every discord message, in any channel:

Nov 21 10:23:07.823 DEBUG message{user_name=REDACTED user_id=REDACTED message_id=1176588622951088150}: galileo::handler: not allowed to send messages in this channel message_info.guild_channel=GuildChannel { id: ChannelId(962034051991097424), bitrate: [SNIP]

I haven't figured out how to shut off those debug logs, so we'll just put up with them for now.