fix memory leaks inside of content_migrations

fixes CNVS-15390

There were a few @model.on events being created in the afterRender of a
few backbone views. These were moved into the initializers so it only
happened once.

Test Plan
Create a migration
Make sure you can still search for a course when doing a migration
Make sure that the progress bar on each migration is still working
Do a little dance.

Change-Id: I188edae3ba3f8b0b7878e5c189ef12e4b1a423de
Reviewed-on: https://gerrit.instructure.com/40835
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Sterling Cobb <sterling@instructure.com>