implement an overflow channel mode

[slingamn]

See discussion on #1238. People who fail to join the channel for any reason (bans, registered-only, user-limit) would be forwarded to this channel. The typical mode for this is +f.

[slingamn]

On freenode, at least, banned users are not forwarded.

[slingamn]

This is +L on inspircd and unreal.

The numeric for this is apparently 470:

:egan.freenode.net 470 c_123456789 #test_01234 #test_12345 :Forwarding to another channel

[slingamn]

It may be necessary to validate that the target channel exists and the mode-setter is an op there?

[slingamn]

At most one forward should be allowed.

[slingamn]

This is needed for an Atheme deployment (so we'll need an atheme2json.py update too).

[slingamn]

The mode letter to use is unresolved. Charybdis uses +f for our semantics, Unreal uses +L for them, Inspircd uses +L but only forwards when the user limit is exceeded.