fix observer link to student modal

test plan:
* create a course with at least one student
* create an observer
* through the cog menu, open the 'Link to Students' dialog

* click on the icon in the right-hand side of the search box
* should dropdown to reveal students in the course
 (rather than showing "No Results Found")
* pushing the down key with an empty list should not cause
 a javascript error

* also confirm that the "Link to Students" option does not
appear when the course is concluded

fixes #CNVS-4204 #CNVS-12820

Change-Id: I7666b89773841c061bd755318915df9540270775
Reviewed-on: https://gerrit.instructure.com/34219
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>