Firebase user login

[Piotr Migdał]

Firebase user login does not seem to work:

Before release, it would be great if we could:

• make it work (at least one way to login)
• display somewhere login status (for all screens, some small text in one of the corners, e.g. Click to log in (and directed to the login page) vs Playing as [name/email] (link to the published levels)
• this listing of saved levels, with a way to make it public

All of such things seemed to work to some extend (at least the login part).

[Jakub Jan Strebeyko]

We figured out point number one, here is a PR for point number two.
I will proceed to point number three on the same branch

edit: done