Add an importer for slack groups

[mrchrisadams]

I'm in a number of slack groups, where we have introduction channels.

In free slacks, you can lose this, and it's useful to make it easier for groups to find each other, by letting users search by name or tags inside the group.

You can could sign in using slack, but there is no support for this sign in process yet.

Would be nice though, although it's probably better to have it an explicit opt-in in the slack, rather than doing a blanket import.

Here's a sample snippet of code to query for users in slack, as a starting point translating it to to js, say:

https://gist.github.com/mrchrisadams/4360a645a9596f046876b6dd083ea19c

[mrchrisadams]

If we wanted to support sign in via slack, it would be a case of implementing this on the serverside, then storing that data in the whatever persistence layer we choose (currently firebase's realtime db).

http://www.passportjs.org/packages/passport-slack/