fix: Avoid duplication of placeholder checks for locked versions

[fsbraun]

Description

djangocms-versioning adds default_checks to placeholders to avoid changing content of published objects. The checks for locked versions were added for every versioned model, but only need adding once.

This PR moves adding the check for locked versions from cms_config.py to apps.py, where it is only done once.

Related resources

• [BUG] Adding a plugin to an apphooked page as admin raises permission error #394
• #...

Checklist

• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.60%. Comparing base (f90c5b2) to head (0094662).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   90.88%   91.60%   +0.71%     
==========================================
  Files          72       72              
  Lines        2546     2573      +27     
  Branches      361      366       +5     
==========================================
+ Hits         2314     2357      +43     
+ Misses        168      153      -15     
+ Partials       64       63       -1     

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[marksweb]

LGTM 👍