Make the deletion-related messages for courses and institutions clear

Update the master messages related to deletion of courses and
institutions to consistently use the words "delete" and "deletion"
instead of the more ambiguous "remove" and "removal", and clarify
the effects of deletion with respect to enrolled students.

Change-Id: Ic6c19dd0b8b8745af70ee3eb55180630e2653cc9
Merge "Make the deletion-related messages for courses and institutions clear"