Skip save if no data changed

[didoda]

This provides an enhancement in saving object data.
BEM already verify form data and remove unchanged. This betters this part by handling categories and tags.
This also skips save if no data has changed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.69%. Comparing base (a204013) to head (429a6b8).
Report is 45 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1153   +/-   ##
=========================================
  Coverage     99.69%   99.69%           
- Complexity     1270     1275    +5     
=========================================
  Files            95       95           
  Lines          4891     4906   +15     
=========================================
+ Hits           4876     4891   +15     
  Misses           15       15           

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