Accessible textual alternative for PDF logo

[robinmolen]

Part of #4622 #5047

Changes

The previous textual alternative, using aria-label, wasn't targetable by screen readers (tested using the macOS VoiceOver screen reader). By providing the textual alternative as real content the screen reader can find and read the content.

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

• Dockerfile/scripts

  • Updated the Dockerfile with the necessary scripts from the ./bin folder

• Commit hygiene

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.67%. Comparing base (1658eca) to head (d9b2299).
Report is 65 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5052   +/-   ##
=======================================
  Coverage   96.67%   96.67%           
=======================================
  Files         765      765           
  Lines       26131    26131           
  Branches     3407     3407           
=======================================
  Hits        25262    25262           
  Misses        606      606           
  Partials      263      263           

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[vaszig]

Tested this visually but had difficulties on testing this with my screen reader (orca).