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.
feat(client): Start to add the account locked screen.
Update the "account is locked" error code to 121.
Start to add the complete_unlock_account and unlock_account_complete screens.
Remove `preventDefaultThen` from most invocations of `validateAndSubmit` and add the decorator directly to `validateAndSubmit`
Revert "Remove `preventDefaultThen` from most invocations of `validateAndSubmit` and add the decorator directly to `validateAndSubmit`"

This reverts commit 3d982df6e54e345f6e2165977d8037715086e963.
Pass the account locked email via ephemeral data.
Fix the account locked tests.