cleanup(test): Use the `fillOutSignUp` functional helper to reduce duplication.
fix(server): Allow the 500 and 503 pages to be iframed.

fixes #1978
Merge pull request #1979 from mozilla/issue-1978-iframe-error-pages

fix(server): Allow the 500 and 503 pages to be iframed.
Merge pull request #1971 from mozilla/cleanup-sign_up-functional-test

cleanup(test): Use the `fillOutSignUp` functional helper to reduce duplication.
fix(client): OAuth/redirect users who paste the verification link into the original tab should be redirected to the RP.

Save a bit to sessionStorage whenever the user signs up or initiates a password reset. When the verification tab opens, it looks for this bit. If the bit is set, that means the user overwrote the signup/reset password tab with the verification tab and the user should redirect to the RP when the OAuth flow completes. If the bit is not set, this is either another tab or another browser, and the user should *not* go back to the RP.

fixes #1816