make files read as buttons rather than links
make files read as buttons rather than links

because they behave like buttons semantically (they open a preview
modal rather than navigating to another page)

test plan:
 - go to a files page wtih Safari/VO
 - files should be read as buttons rather than links
 - this is also true of folders on the right pane
 - accessibility issues with the folder tree on the left
   will be addressed separately

fixes ADMIN-2619

Change-Id: I59df2d7e34c6098bb001a121906ddc97f3153186
Reviewed-on: https://gerrit.instructure.com/192431
Tested-by: Jenkins
Reviewed-by: Carl Kibler <7c2b40129dd885a147d5b2dda56dda016b9a8f57@instructure.com>
QA-Review: Daniel Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>
Product-Review: Daniel Sasaki <ae19fabf7fe19dff310008c9b6a4d2ec84d311d8@instructure.com>

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