Revert "Merge pull request #335 from mope/edit-handlers-unit-tests"

This reverts commit 7ad6be82f3913893dcd82dd2af904da4c654c3ff, reversing
changes made to 4c77a4b18a8fef9abd3bcc1fccaf5627f1604522.
Merge branch 'master' into fix-edit-handlers-test

Conflicts:
	wagtail/wagtailadmin/tests/test_edit_handlers.py