add migration_selection lti extension

this commit lets canvas remember that an LTI has this extension.
launching and using LTI for content migrations will happen
separately.

refs LOR-20

test plan:
 - configure an LTI tool with XML that includes the
   migration_selection extension
 - the tool should show up in the tool list with the "Migration
   Selection" tag.

Change-Id: I880125515a32af1b9a39fbd1787617bceae74f50
Reviewed-on: https://gerrit.instructure.com/21442
Reviewed-by: Jake Trent <jaketrent@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>