combine login and registration page

[mmnoo]

as discussed in https://github.com/publiclab/wherewebreathe/issues/7 Potentially phase II

[jywarren]

Another thing that you can do is rely on the login form on every page -- although on mobile it takes a couple clicks -- and have the "log in" alternative just open that form. That later will make it easier to redirect people back to the page they were on when they logged in.