preserve discussion focus when pinning/closing for comments
preserve discussion focus when pinning/closing for comments

test plan:
 - go to the discussions index page
 - use the keyboard with the gear menus to pin/unpin discussions
   and close them for comments
 - ensure when an item is moved out of the list, the previous
   item is focused, or if there is no previous item, the list
   header is focused
 - if an item doesn't move (such as when opening a pinned
   discussion), focus stays on the gear

hopefully fixes CNVS-28556

Change-Id: Icc4ecf2842e08ca8b7f3ef12292af358578f522f
Reviewed-on: https://gerrit.instructure.com/78367
Tested-by: Jenkins
Reviewed-by: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>
QA-Review: Deepeeca Soundarrajan <5bcd4b623f00b7fc2942cd1c65c245b30a48a61f@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

roxolan/canvas-lms,venturehive/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,dgynn/canvas-lms,HotChalk/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,HotChalk/canvas-lms,fronteerio/canvas-lms,fronteerio/canvas-lms,instructure/canvas-lms,roxolan/canvas-lms,fronteerio/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,roxolan/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,matematikk-mooc/canvas-lms,grahamb/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,dgynn/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,matematikk-mooc/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,SwinburneOnline/canvas-lms,roxolan/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,matematikk-mooc/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,dgynn/canvas-lms,dgynn/canvas-lms,fronteerio/canvas-lms