add option to open external url module items in new tab

test plan:
* should be able to create and edit external url module items
 with an option to require the urls to be opened in a new tab

* on the modules index page, clicking on these items should
 directly link to the url in a new window, but also redirect
 the open canvas page to the module item page (so to register
 progression, and show the sequence footer)

* navigation through the sequence footer should be unaffected

* should work with draft state enabled and disabled

fixes #CNVS-12579

Change-Id: Ic0b33014acd43f72b05258705a87221c4b8d61c3
Reviewed-on: https://gerrit.instructure.com/35123
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>