Standardise channel management services

[DanielOaks]

So, ircv3/ircv3-specifications#276 does the job of introducing a way to basically expose NickServ methods natively (allowing clients to build pretty interfaces for them). This issue is about doing the same thing for ChanServ and channel management more generally.

My plan is:

1. Get #276 accepted, or at least get other implementers on-board with it so we know the API is stable and relatively well-received.
2. Introduce a channel management PR that's... basically the same thing, using an extremely similar API and command/subcommand set, but aimed at registering channels instead.

Pretty simple. I won't introduce a channel management PR before the account management one goes ahead, because... plain and simple, I don't want to bother keeping two separate PRs in-sync in terms of the API they expose, and also don't want to run into the situation where one gets standardised in one state and the other gets standardised in a slightly different state.