Some fixed strings are now translatable

[svandeneertwegh]

Description

constants.py = 4 fixed strings which dit not parsed through _() models.py = 2 date fields which did not had verbose_name

• [ ] I have opened this pull request against master
• [ ] I have added or modified the tests when changing logic
• [ ] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [ ] I have read the contribution guidelines and I have joined #workgroup-pr-review on Slack to find a “pr review buddy” who is going to review my pull request.

[codecov[bot]]

Codecov Report

Merging #310 (3f9705a) into master (b77ef6f) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #310   +/-   ##
=======================================
  Coverage   89.87%   89.87%           
=======================================
  Files          68       68           
  Lines        2242     2242           
  Branches      299      299           
=======================================
  Hits         2015     2015           
  Misses        172      172           
  Partials       55       55           

Impacted Files	Coverage Δ
djangocms_versioning/models.py	94.85% <100.00%> (ø)

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[fsbraun]

@svandeneertwegh Can you please run django-admin makemessages --all in the djangocms_versioning folder and include the update po files in the PR?