Rewrite dirtyfields with FieldTracker

We already had django-model-utils as dependency, which provides the same functionality as django-dirtyfields.

added part::backend label

mentioned in issue #375 (closed)

added 3 commits

• 0f6ab87f - Rewrite dirty field tracking in Person and Group using FieldTracker
• 5863449a - Remove dirtyfields dependency
• 4d067144 - Fix reverse logic bug in checking dirty state in Group.save

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added 1 commit

• 60a0960b - Rewrite dirty field tracking in Person and Group using FieldTracker

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merged

mentioned in commit 26fabd33