Implement field differ and sync for DiffedContent

[aapomm]

Spec

Users might want to update specific fields only, and they may or may not want to sync all of the changes.

Proposed solution
In the Issue Library widget, users will be able to see all the fields that have been changed. Each will serve as a link to a modal where the user will be able to see the diff for the field in question.

In the issue#show IssueLibrary widget:

• Update the diff modal to support the per-field diff
  • Update the DiffedIssue service class
    • Checks if the field of the issue is outdated against the associated field of the entry
    • Uses Differ to show the difference between the field of the issue and the on of the entry
• Implement a one-click action to sync the fields

Check List

- [ ] Added a CHANGELOG entry