fix: deletion of version objects blocked by source fields

[fsbraun]

Description

source fields refer to Version objects a version was originally derived (copied) from. They are declared with on_delete = models.PROTECT. This blocks deletion of versioned objects (such as pages) which have sources.

If DJANGOCMS_VERSIONING_ALLOW_DELETING_VERSIONS is set to True, on_delete = models.SET_NULL removes the block and a user with permissions to delete Version objects can delete pages.

Remark: This does not affect the models.PROTECT setting for the Version.content.

Related resources

• This change in model is reflected in the existing migrations but does not affect database constraints (see Django docs). Toggling the settings will therefore not create migrations.
• A test for missing migrations has been added. It needs to run as the first test for otherwise versioning's proxy models might create an incorrect migration.
• This test revealed that two verbose names of fields were not referred to in the migrations. This is fixed.

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 #320 (2592545) into master (6d062b2) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
+ Coverage   89.96%   89.99%   +0.02%     
==========================================
  Files          68       68              
  Lines        2253     2259       +6     
  Branches      300      301       +1     
==========================================
+ Hits         2027     2033       +6     
  Misses        171      171              
  Partials       55       55              

Impacted Files	Coverage Δ
djangocms_versioning/migrations/0003_version.py	100.00% <ø> (ø)
...cms_versioning/migrations/0015_version_modified.py	100.00% <ø> (ø)
djangocms_versioning/conf.py	100.00% <100.00%> (ø)
...gocms_versioning/migrations/0014_version_source.py	100.00% <100.00%> (ø)
djangocms_versioning/models.py	95.05% <100.00%> (+0.13%)	:arrow_up:

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