Improve UX for setting display name

[santakadev]

From #40

When a second user opens the same session but before he introduces its nickname, it appears in the Users list as Anonymous. This could be confusing for the users already in the session because they could think that someone has joined and can actually see the contents of it (something not true yet).

However, I wouldn't vote for changing this behaviour grimacing. Instead, I would go for leaving the user land right in the editor of an existing session and letting him edit his preferences afterwards. That is, not asking his nickname before joining the session. The main reason would be to reduce the friction with the user to the bare minimum while joining a session. This is something similar to what http://zoom.us does while joining an existing conference session

If we go for this approach, it'll be definitely useful to list new session joiners as Anonymous until they set their nickname slightly_smiling_face