Fix date picker not appearing

after the jquery upgrade, the date picker on
the calendar page disappeared. The z-index
on the element was being overriden inconsitently

fixes LF-1319
flag=none

test plan:
- go to the calendar page
- with each view (month/week/agenda) open the
  date picker by clicking the date on the left
  side of the header
- ensure the date picker appears and functions as normal

Change-Id: I2adf124f537be7699dcac11a7f46d4e122c22e27
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/341798
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Robin Kuss <rkuss@instructure.com>
QA-Review: Robin Kuss <rkuss@instructure.com>
Product-Review: Sarah Gerard <sarah.gerard@instructure.com>

app/stylesheets/vendor/jqueryui/_jquery.ui.datepicker.scss

@@ -75,7 +75,7 @@
 
 
 #ui-datepicker-div {
-  z-index: 9999;
+  z-index: 9999 !important;
 }
 
 .ui-datepicker-header.ui-widget-header {