adds aria-{invalid,describedby} annotations for WTForms validation errors

[cfm]

Status

Ready for review

Description of Changes

Closes #6238 (towards #5972) by adding aria-invalid and aria-describedby annotations where possible for validated WTForms-based forms. This is admittedly a pretty minor improvement; see #6238 for details on all the places where even this improvement is either not applicable or not possible in the current interfaces of the Source and Journalist Interfaces.

Testing

1. Log in to the Journalist Interface as an administrator.
2. Begin adding a user and enter invalid values for each field.
3. Click Add User.
4. When the form comes back with validation errors, view source and observe that each invalid field is marked aria-invalid and has an aria-describedby annotation pointing to its validation errors. (These annotations will also be used by assistive tech.)

Deployment

No deployment considerations.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

[codecov-commenter]

Codecov Report

Merging #6240 (d72e0dc) into develop (37f969e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6240   +/-   ##
========================================
  Coverage    84.09%   84.10%           
========================================
  Files           60       60           
  Lines         4220     4222    +2     
  Branches       509      509           
========================================
+ Hits          3549     3551    +2     
  Misses         550      550           
  Partials       121      121           

Impacted Files	Coverage Δ
securedrop/journalist_app/__init__.py	90.17% <100.00%> (+0.08%)	⬆️
securedrop/source_app/__init__.py	90.62% <100.00%> (+0.09%)	⬆️

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[cfm]

Thanks for reviewing, @legoktm. I'll revise this week.

[cfm]

Thanks for the review, @legoktm. Revised accordingly, including the suggested minor refactoring to Jinja's do expression.

[legoktm]

LGTM, test plan still checks out.