Kill <channels> in favour of (already existing) <connect:maxchans> and (new) <oper:maxchans>.

This makes the following changes:

- Removes the `<channels:users>` setting in favour of the *already existing* `<connect:maxchans>` setting.
- Replaces the `<channels:opers>` setting and the `channels/high-join-limit` oper privilege in favour of specifying a higher join limit in the `<type>` or `<oper>` block.

Side note: I think we should implement an override of On005Numeric which takes a `ConnectClass*` so that we can deal with connect class specific tokens like MAXCHANNELS.