add migration external tool extension ui

An external tool configured for the migration extension will
show up on a course's content import page and allow you to
select a file to migrate from the LTI launch.

fixes LOR-20

Test Plan:
 * Configure an external tool that has the migration_selection
   extension
 * The new tool should show in the dropdown on the content
   import page.
 * Run some migrations through the external tool.

Change-Id: Iea16a7e32145e6799c551816e6f70a73f2a8c3d0
Reviewed-on: https://gerrit.instructure.com/21443
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jake Trent <jaketrent@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>