Improve CSRF

[jwag956]

Added testing to /tf-setup - there wasn't any CSRF issue - all working.

CSRF handling is complex and there are few unit tests.

• Added @pytest.mark.csrf to make it easier to turn on and test CSRF w/o lots of boilerplate
• Added tests and improved many templates to show CSRF errors - mostly for developers - but otherwise CSRF errors tent do just disappear and are difficult to debug
• Found issue with WTFforms with the new form-level errors - it uses a None key - which, if there are multiple errors, isn't sortable by Flasks default JSON serializer. Filed issue and now change if from None to ""
• Fixed issue in webauthn with CSRF errors causing exceptions - added tests.
• In the case of CSRFprotect() (the app configuring CSRF for the entire app) a CSRF error would raise an exception which would always return an HTML response - added code to return a JSON response if desired.

closes #905

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a2ae67c) 98.38% compared to head (0363354) 98.39%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #908 +/- ## ======================================= Coverage 98.38% 98.39% ======================================= Files 34 34 Lines 4465 4484 +19 ======================================= + Hits 4393 4412 +19 Misses 72 72 ```

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