Afform - Add undo/redo buttons to the Admin UI

[colemanw]

Overview

Prevents unsaved work being lost in Form Builder.

Before

No undo button, no warning about unsaved changes.

After

Tracks undo history up to 20 revisions.
Warns of unsaved changes before leaving the screen.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[monishdeb]

• General standards

  • (r-explain) PASS
  • (r-user) PASS
  • (r-doc) PASS
  • (r-run) PASS: I have tested the patch in the test-build generated http://core-23648-3bsiu.test-1.civicrm.org:8001

• Developer standards

  • (r-tech) PASS
  • (r-code) PASS
  • (r-maint) PASS
  • (r-test) PASS