Fix form.next for newer authentication mechanisms - including two-fac…

[jwag956]

…tor.

Propagating 'next' when part of the request query string was working - but FS also supported 'next' as a hidden field in some endpoints such as /login and /register. form.next wasn't being propagated with 2FA. Also - newer authentication endpoints - /us-signin /wan-signin didn't have the 'next' hidden field.

Also - there were NO tests for form.next.

Fixed all that, added tests, combined all 'next' calculations in a single utility.

closes #853

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
flask_security/oauth_glue.py	98.86% <ø> (ø)
flask_security/tf_plugin.py	99.16% <100.00%> (ø)
flask_security/unified_signin.py	98.75% <100.00%> (ø)
flask_security/views.py	98.76% <ø> (ø)
flask_security/webauthn.py	98.58% <100.00%> (+<0.01%)	⬆️
flask_security/utils.py	97.58% <96.66%> (+0.19%)	⬆️

📢 Thoughts on this report? Let us know!.