fix changes_display_dict to display added/deleted objects for m2m updates

[a-cesari]

In one of my projects I needed to use the changes_display_dict function in a Django template to display changes of a m2m field.
Apparently for such field the changes_display_dict function is not storing the added/deleted object while the changes_dict function does. The changes_display_dict function just store a list like this: ['type', 'operation', 'objects'] .
See screenshots attached.

I slightly modified the changes_display_dict function to also store the list of added/removed objects. The result is the following:

I hope you could consider merging this if appropriate. Happy to implement any changes/improvements you might suggest on my current proposal.

[codecov]

Codecov Report

Merging #464 (6403f34) into master (1674aca) will decrease coverage by 0.57%.
The diff coverage is 16.66%.

❗ Current head 6403f34 differs from pull request most recent head 7bf1798. Consider uploading reports for the commit 7bf1798 to get more accurate results

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
- Coverage   92.74%   92.17%   -0.58%     
==========================================
  Files          26       26              
  Lines         786      792       +6     
==========================================
+ Hits          729      730       +1     
- Misses         57       62       +5     

Impacted Files	Coverage Δ
auditlog/models.py	85.89% <16.66%> (-1.77%)	⬇️

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[aleh-rymasheuski]

@a-cesari, thanks for your patch! I indeed missed changes_display_dict logic when implementing m2m tracking because it's a feature which I don't use myself.

Could you please cover the added logic with a test? Can you also add a changelog entry for this feature?