Feature/4717 accessibility improvements aria

[robinmolen]

Closes #4717

Changes

The site logo now provides a better textual alternative, the error message element now has the appropriate role and the PDF's are now screenreader friendly :)

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.55%. Comparing base (7b20acc) to head (cd6f874).
Report is 115 commits behind head on master.

Files with missing lines	Patch %	Lines
src/openforms/formio/rendering/default.py	88.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4747      +/-   ##
==========================================
- Coverage   96.56%   96.55%   -0.01%     
==========================================
  Files         746      746              
  Lines       25174    25212      +38     
  Branches     3310     3324      +14     
==========================================
+ Hits        24309    24344      +35     
- Misses        601      603       +2     
- Partials      264      265       +1     

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