Announcement/Discussion search field has label

fixes CNVS-14356

Added a label tag with the requisite text for NVDA
Fixed white space
Fixed aria-label

Test Plan
* Navigate to the search box in a discussion (/courses/#/discussion_topics/#)
* Verify that NVDA reads "Search entries or author"

Change-Id: I1827faeb21e31f8ab63024edea4c55bcc70eef4a
Reviewed-on: https://gerrit.instructure.com/43398
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>