use tz() to get current offset

instead of icky $('#time_zone_offset')

test-plan:
 - have the current user's profile timezone differ from the system
   timezone (e.g. profile=Alaska, browser=Mountain)
 - create an assignment throught the UI as that user with a specific due
   at (e.g. 10pm March 14)
 - due at should be interpreted relative to profile timezone when
   saving, confirm by comparing to value in database
   (e.g. interpreted as 10pm AKDT; should end up as 6am March 15 UTC in
   database)
 - load assignment edit page
 - assignment should display in profile timezone (e.g. should display as
   10pm, not 12am)

Change-Id: Icd8e0b60707bd1bfdc2db8fa9195940bd993bdc7
Reviewed-on: https://gerrit.instructure.com/31545
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>