Show (and join from) channel list

[raucao]

[raucao]

Just FYI: on Freenode, that's a bit more than 10K channels, so only makes sense when also implementing search with that from the get go.

[raucao]

I added #78 as a stopgap, so one can join channels on newly created spaces.

[raucao]

https://github.com/sockethub/sockethub-platform-irc/issues/23