Removed role of button on headings

fixes CNVS-16979

Removed the role of button on the date range, calender, and undated
items heading. Changed Calendar and Undate Items headings to be h3
elements instead of h4.

Test plan:
* Log in and go to the calendar agenda view
* Using a screen reader make sure that when navigating through the page
  the date range, Calendars, and Undated Items headings are treated as
  headings and not buttons

Change-Id: Ic80201179bae77647aebdb312f5aab4dd2d27b26
Reviewed-on: https://gerrit.instructure.com/44995
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>