Select icon in conversations dropdown

fixes CNVS-14436

This fixes the icon not showing up correctly in the conversations
dropdown

Test Plan:

- Go to Inbox
- Click on "All Courses"
- Make sure a checkmark icon shows on the selected course
- Also click on "Compose a Message"
- Make sure the checkmark also appears in the Courses dropdown here

Change-Id: I720424dcf8df0db10141ba06e6d8d53266939fea
Reviewed-on: https://gerrit.instructure.com/39153
Reviewed-by: Chris Hart <chart@instructure.com>
Product-Review: Janelle Seegmiller <jseegmiller@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>