add icon to mark all as read option on discussion show page
add icon to mark all as read option on discussion show page

fixes CNVS-5886

test plan:
 - go to a discussion and make sure the icon appears in the gear
   menu next to the "mark all as read" command.

Change-Id: I606477ceb0e2e4dabfe2e9e5d59cad6788274848
Reviewed-on: https://gerrit.instructure.com/21081
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Cam Theriault <b4133c36b6d893d0872cc09e87b536d16aa2d241@instructure.com>
Reviewed-by: Joel Hough <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@instructure.com>
Product-Review: Jon Willesen <eb5ce5e6a76c243326fecfa94534101600f87bd9@instructure.com>

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