Update youtube cog for several youtube channels.

[SylteA]

Simple version is we update the config for another youtube channel, but then what if we want to monitor another youtube channel?

I'd like for us to remove the youtube related config variables and create a new database model for monitored channels.

The model could contain something like:

• guild_id
• channel_id
• youtube_id
• role_id: optional[int] - if role_id is None then we post without mentioning a role.