MongoEngine changes to support mongodb 2.6

Summary:
Remove safe=True in mongoengine calls. Safe mode is deprecated and
rolled into write concern in 2.6.

Test Plan: Ran unit tests over local server & clicked around. Everything worked

Reviewers: tarek, josh, jim, david, vitaliy, jack

Maniphest Tasks: T11511

Differential Revision: http://phab.wish.com/D18696