fix: Copy language menu

[fsbraun]

Description

djangocms-versioning patches the toolbar and rebuilds the copy language menu to copy from the latest (potentially draft) version of the other language.

If no version of another language existed the toolbar threw an exception when trying to get the link for the action. With the toolbar raising an exception the page content becomes uneditable.

This PR greys out languages which do not have any published or draft version available to copy a language from.

Checklist

• [x] I have opened this pull request against master
• [x] I have added or modified the tests when changing logic
• [x] I have followed the conventional commits guidelines to add meaningful information into the changelog
• [x] 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 #299 (f9e8c91) into master (e6a33ea) will decrease coverage by 1.22%. The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #299      +/-   ##
==========================================
- Coverage   87.61%   86.39%   -1.23%     
==========================================
  Files          29       23       -6     
  Lines        1591      801     -790     
  Branches      260      106     -154     
==========================================
- Hits         1394      692     -702     
+ Misses        141       82      -59     
+ Partials       56       27      -29     

Impacted Files	Coverage Δ
djangocms_versioning/cms_toolbars.py	94.36% <50.00%> (-1.35%)	:arrow_down:
djangocms_versioning/helpers.py	90.90% <100.00%> (ø)
djangocms_versioning/admin.py
djangocms_versioning/models.py
djangocms_versioning/forms.py
djangocms_versioning/cms_config.py
djangocms_versioning/__init__.py
djangocms_versioning/conf.py

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