Add Pusher private channel [4h]

[amitaibu]

[amitaibu]

@rgrempel how much time do you think it will take to copy from other internal projects?

[rgrempel]

How much time would you like it to take :-)

It's not actually totally finished on the internal project you're thinking of, because I kind of tangled it up with other (nice) changes to the process of reacting to login/logout and dealing correctly with auto-login and failures to login and pages which require login and "redirect" you once you've successfully logged in etc. Though the "private channel" aspect of it is basically done, and could be extracted from those other changes.

In fact, I probably should extract it (also on the internal project) and deal with the entangled issues separately, since they are really distinct issues (also useful to handle nicely someday, but distinct).

So, say about 3 hours to extract the purely private-channel parts of the existing work and integrate here.

[amitaibu]

Good, I had 4h in mind :)