Make the id for a user fillable

Summary: Allow the unique identifier for a user account to be filled out for the purposes of import in transition, so as to preserve existing customer numbers.

Test Plan: None

Reviewers: #hkccp_developers, machniak

Reviewed By: #hkccp_developers, machniak

Subscribers: machniak, #hkccp_developers

Differential Revision: https://git.kolab.org/D1219