feat: Add preview button to view published mode

[fsbraun]

Description

Currently, there is no way to use the toolbar to get from viewing a published version of a page (or other object) back to its preview. The only option available is "Edit" which will create a new draft or go to an existing draft.

With this PR the view published mode will have an additional button to get (back) to the preview.

I expect it to create more transparency on the modes available with djangocms-verisoning

• Edit mode (as with the core): Visible to staff only
• Preview mode (incldeing draft content): Visible to staff only
• Published mode (only published content): Visible to the outside

Also there are use cases where published versions and the preview of a public version need to be compared:

• The published version only contains public versioned items
• The preview of a public version contains draft versions of, e.g., versioned alias objects

Related resources

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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 #316 (b1875ac) into master (33a28c9) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
+ Coverage   89.88%   89.91%   +0.02%     
==========================================
  Files          68       68              
  Lines        2244     2250       +6     
  Branches      299      301       +2     
==========================================
+ Hits         2017     2023       +6     
  Misses        172      172              
  Partials       55       55              

Impacted Files	Coverage Δ
djangocms_versioning/cms_toolbars.py	95.85% <100.00%> (+0.15%)	:arrow_up:

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