User mention on Page Template creation does not work

Problem Definition

User mentions do not work when used inside the Page Template creation/edit

Steps to reproduce

1. To go Settings > Global Template and Blueprints
2. Create a new one or edit an existing Page Template
3. Try to use a user mention

Expected results

Typing @ brings up the user search. Adding the User mention macro allows us to mention a user.

Actual results

The user search does not come up. The mention macro is broken.