bugfix: don't strip hyphens from media thumbnail URLs

some media ids have dashes in them, but sanitization in the canvas
kaltura client was stripping them, causing video thumbnails to not work
in some cases.

this fixes the sanitization so those ids are handled properly.

Change-Id: I23b4c392dfc79eff4d57bea84c3509a1e6bf3137
Reviewed-on: https://gerrit.instructure.com/39694
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Paul Hinze <paulh@instructure.com>
QA-Review: Paul Hinze <paulh@instructure.com>