simplify time_ago_date processing

there was a one-off unfudge calculation. meticulous manipulation of the
code gives almost the exact same implementation as
unfudgeDateForProfileTimezone, and whose only deviations appear to be
bugs.

however, doing that unfudge (whether adhoc or using
unfudgeDateForProfileTimezone) was only necessary and appropriate when
the date to parse didn't have TZ info embedded (which it sometimes, but
only sometimes, did).

to simplify, just embed a known ISO formatted timestamp in all
time_ago_date elements. then we can parse that and use if for
calculations without any unfudging.

but don't forget to fudge that date before formatting for display (title
attribute and displayed string when > 1 day old).

test-plan:
 - have a student in a course that's posted on a discussion in that
   course
 - as a teacher in that course whose profile timezone does not match the
   system timezone, view the student's course roster page
   (/course/x/users/y)
 - under "Recent Messages", confirm that the friendly timestamps on the
   right for the discussion entry is displayed according to the correct
   (profile) timezone
 - confirm the title attribute (hover) of the friendly timestamp is
   formatted in the profile timezone

Change-Id: Id3385d58a9da02e9ed991ebd59152ca3f5fbd280
Reviewed-on: https://gerrit.instructure.com/31591
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>