add launch_type parameter to external_tools show action

some planned extensions will need to launch their tools in a
manner similar to a module item tool, but with their extension
specific settings. Added the launch_type parameter to the show
action to make this possible.

refs RAN-53, RAN-55

test plan:
 - configure an external tool that has a an integration type
   other than account_navigation or course_navigation.
 - enter the url for the tool in a browser:
     /course/<course-id>/external_tools/<tool-id>
 - this should display an error.
 - add the launch_type parameter to the url:
     ?launch_type=<extension_name>

Change-Id: I1495db4deca8042ddf6f09548b006a0f7741db35
Reviewed-on: https://gerrit.instructure.com/35159
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>