R1:25bbe741f45b

Provide a default non-NULL timezone in the PhabricatorUser class

Summary:
Without this, user creation throws an exception when trying to insert NULL into
a non-NULL field.

Test Plan:
Created a new user.

Reviewed By: fratrik
Reviewers: fratrik, toulouse, aran, jungejason, tuomaspelkonen
CC: aran, fratrik
Differential Revision: 480