Fix bug 866533, 1054559: Update registration page

Supersedes https://github.com/mozilla/kuma/pull/2691 with tests for the sign-in and sign-up pages changed.
Yeah. The Sign Up view tests will test the logic of the different sign-up scenarios, so they are still to do. (As part of the tests for https://bugzilla.mozilla.org/show_bug.cgi?id=1055870)

The tests in this PR are the sign *in* view tests, and the sign *up* template tests for the registration copy included in this PR.