User page improvements

The error will not appear red *yet* because #2558 has not been merged. If we merge both, in either order, the error will be red on production. If it helps, I can also rebase this branch against master after #2558 lands, so that the error appears red when this branch is checked out in Vagrant.

After logging in, the *Verification Sent* page appears, and I can't figure out how to work around it and see the registration page. Any tips?
This should use the action `{{ url('socialaccount_signup') }}` to stay in line with allauth's default templates.