Enchance a11y on dashboard activity feed

Fixes CNVS-14501

Makes the show/hide more and remove options in the dashboard activity
feed show all the time, not just on mouseover, in order to improve
keyboard a11y.

Adds aria-label to the context link informing a screenreader user where
that link is going to go.

Test Plan:

1) Give yourself at least one announcement, conversation message,
   assignment notification, and discussion. Easiest way is to create a
   a course and add an announcement, send a message to yourself from
   another user, add an assignment, and add a discussion.
2) Visit the dashboard (localhost:3000/)
3) Verify that the list of notifications is navigable with the keyboard
   only, because you can see from the get go that the 'show more' option
   and remove/ignore option.
4) Using a screenreader, navigate to the notification heading, then onto
   the course name link. Verify that you hear label text like
   "Visit Course Discussions" or similar. This should help SR users only
   click that option if that's what they want, so they keep looking for
the "Show More" option (it's one link further through the tab flow) if
that's what they want instead.

Change-Id: I0726510a351f16768d98657e9891cb7a1c162be3
Reviewed-on: https://gerrit.instructure.com/41417
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>