Registration form: clear the previous error state on submit

When user enters an unproperly formatted error message, an error message
is displayed. Then when the email address is corrected and the
registration succeeds, the previously displayed validation error is
removed.
Registration form: disables the input and the submit button

So that the user doesn’t get a chance to mess up.
Registration form: complete success message
Extract registration form’s CSS to a separate file
Registration form: show Firebase error code for the unknown erorrs
Registration form: disable form and show spinner on submit

Sometimes Firebase may take a while to respond. This also prevents
multiple form submissions.