Fix: "Show more" link loses the focus on chrome using voiceover
Fix: "Show more" link loses the focus on chrome using voiceover

The "button" role was added to the link, this works better to keep
the design and prevent the voiceover from setting the focus on the
target href

fixes LS-2819
flag = none

Test plan:
- Using chrome and in a normal canvas account go to a course and
create some announcements
- Go to the dashboard and set the dashboard view to “Recent Activity”
- Open VoiceOver and navigate to the “SHOW MORE” link
- Press control+option+space and expect the focused element to be
the first announcement in the list

Change-Id: Ie176b1326792c62ceedc509eb38c1a1e8e3c4c86
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/277801
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Product-Review: Jonathan Guardado <94d0a13686225e6d8af5347ed17f9cc0c2643995@instructure.com>
Reviewed-by: Luis Oliveira <4ec7209c0a6350a2c3877fe0e8e120f972fc692a@instructure.com>
QA-Review: Luis Oliveira <4ec7209c0a6350a2c3877fe0e8e120f972fc692a@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms