feature(elm): Add notification on player joining the room

[mrapacz]

Summary
I put the 'blocked' label as we're still waiting for a tool that would let us display notifications in an elegant manner (by elegant I mean different than Debug.log).

There are two different types of messages depending on whether you join the channel or someone else. To make sure if it's you or someone else I compare values of currentUser.name and the name of user who has just joined.

Of course it's not the proper way to do it, it will be faulty as soon as we get two users with the same nicknames in the same room. (or maybe nicknames should be unique?). Anyway I prefer that a better way to identify players would be by using their encoded ids (see: #56).
Related issues
closes: #30

Test plan
Join a single room with two users and watch the notifications written to the log.

[mrapacz]

I thought we were going to use toasty notifications for people joining the room, but since it's not the case, I'll just fix conflicts and merge it with master.