resolve circular dependency between App and UserModel

Summary: When App.user was added to App it meant that UserModel was loaded within the App definition but also requires App itself, which is undefined at that point. This fix makes UserModel use the require() function to fetch App when needed. Fixes T818

Test Plan:
1. Go to settings/users
2. Click edit on any of the users.

Reviewers: shadowhand, rjmackay

Reviewed By: rjmackay

Maniphest Tasks: T818

Differential Revision: https://phabricator.ushahidi.com/D384