fix now-line placement in weekly calendar
fix now-line placement in weekly calendar

test plan:
 - the yellow "now" line should be positioned according to the
   current time in the weekly calendar, and not
   (a) shifted down a lot with a really big browser window, or
   (b) shifted up a bit when the current time is close to the
       end of the hour

fixes LS-1177

Change-Id: I38b95b069f55bee5c384231f02de4ae2271eca77
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/242570
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms