fix: Minor usability improvements

[fsbraun]

Description

Ajax error message in page tree

Since django CMS 4.1.0rc1 the page tree shows error messages returned by ajax post requests such as toggling navigation status of a page content object.

No additional message needs not to be sent to the front end. It only results in the error message shown again after a reload.

Edit button renamed to "New Draft" if the action creates a new draft (as opposed to go into editing mode of an existing draft)

To avoid accidental creation of many draft objects by clicking the Edit button in the toolbar this PR renames the Edit button to "New Draft" if by clicking it a new draft will be created.

Menu item "Discard changes" in versioning menu

This replaces v3's "Revert to published" function by calling the discard view (without the need of going through "Manage versions")

Replace deprecated icons

• cms-icon-copy-> cms-icon-manage-version
• cms-icon-layers -> cms-icon-compare
• Use newly provided cms.icons.css resource

Related resources

• https://github.com/fsbraun/django-cms/blob/1b79a030dbc9b09543d0e7cbe77143397b8ab70a/cms/static/cms/js/modules/cms.pagetree.js#L905-L910
• https://github.com/django-cms/django-cms/pull/7494

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 #317 (212dacf) into master (65f6da9) will increase coverage by 0.05%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
+ Coverage   89.91%   89.96%   +0.05%     
==========================================
  Files          68       68              
  Lines        2251     2253       +2     
  Branches      300      300              
==========================================
+ Hits         2024     2027       +3     
+ Misses        172      171       -1     
  Partials       55       55              

Impacted Files	Coverage Δ
djangocms_versioning/admin.py	88.76% <ø> (ø)
djangocms_versioning/cms_config.py	80.59% <ø> (+0.30%)	:arrow_up:
djangocms_versioning/helpers.py	91.11% <ø> (ø)
djangocms_versioning/indicators.py	80.00% <ø> (ø)
djangocms_versioning/cms_toolbars.py	95.95% <100.00%> (+0.09%)	:arrow_up:

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