Add REST API method to list online users in a room

[wtsarchive]

@RocketChat/core

Adds a REST API method for listing online users in a room. Useful to show room activity on external sites. Example:

curl http://localhost:3000/api/rooms/GENERAL/online

{
   "status": "success",
   "online": [
      "test",
      "admin"
   ]
}

[sampaiodiego]

@wtsarchive can you please make a rebase with our latest code from develop?

seems ok to me to merge it =)

[wtsarchive]

@sampaiodiego Done!

Ah no, I thought "Update branch" button would do a rebase. Give me a sec to do a proper rebase.

[wtsarchive]

Ok, now it's properly rebased.