Fix loading WordPressAuthenticator xibs

[jtreanor]

Fixes #11444

It relies on the changes in wordpress-mobile/WordPressAuthenticator-iOS#78. That should be merged and released before this PR.

To test:

1. Open the app logged out.
2. Start the email signup flow.
3. Enter a new email address and confirm the signup via the magic link.
4. On the signup epilogue screen, tap the username field to try to change your username.
5. It doesn't crash.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[jtreanor]

Thanks @astralbodies! It's now updated to use 1.3.1 of WordPressAuthenticator. There are no changes other than pointing to the actual release so I will merge when green.