Fixes bug in User Profile form

Fixes bug documented here: https://jira.dosomething.org/browse/DS-438

Refs https://jira.dosomething.org/browse/DS-386

* Adds a new function`_dosomething_user_global_display_fields` which hides/displays fields in both the Profile and Register forms

* Refactors `_dosomething_user_register_display_fields` to hide/display fields specific for the Register form