[#4298] Broken admin links and certificate label in email digest

[vaszig]

Closes #4298

• Handle empty labels in certificates
• Replace admin links with the absolute ones

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.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

All modified and coverable lines are covered by tests ✅

Project coverage is 96.26%. Comparing base (9b49cd1) to head (6181865).
Report is 729 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4317   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files         731      731           
  Lines       23718    23723    +5     
  Branches     2795     2795           
=======================================
+ Hits        22833    22838    +5     
  Misses        616      616           
  Partials      269      269           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.