Improve signup form error messaging and handling

This refs bug 1036129 and 1049982 by offering a Persona login link if a Persona account has been found in our database, then redirecting to the connect workflow for adding a Github account.

Since the Github permissions have been granted already on Github's side this will directly continue to connecting and then redirect to the profile edit page.

- [x] Get feedback from @stephaniehobson about UX
- [x] Improve UI to have a proper panel/modal according to UI design from Holly
- [x] Figure out how to properly redirect to the originating URL when doing the Github > Persona > <URL> steps
- [x] Show warning notification if an account exists with one of the verified Github emails fetched from Github
- [x] Shown default notification if no account exists like the point above
- [x] Improve error message when submitting the form an a username is entered that already exists
- [x] Show a "username already exists" error on the first page load when coming back from Github
- [x] Improve inline form error in case an email address has been used before on form submission including a link to to the Persona->Github workflow
Can this be `User.objects.get()` so it returns a single object? That way the template doesn't need to do `matching_user.0`.