make kaltura thumbnail fetching always use https

there were several problems with the existing implementation:
 - the conditional for selecting http/https was backwards
 - kalturaSettings.secure_resource_domain is not an option

since kaltura is always available over https, and since resource_domain
is a required kaltura setting, we can drastically simplify the URL
building and remove all conditionals from the process.

this also adds a basic qunit smoke test for the mediaCommentThumbnail
functionality.

test plan:
 - visit existing production URL (which always uses https) with a
   kaltura video. note that when the thumbnail for the video loads, your
   browser shows a mixed content warning.
 - visit an environment with this changeset over https
 - pull up or create a page with a kaltura video
 - note that the thumbnail is loaded over https and no mixed-content
   warning occurs

Change-Id: Iffcdd382f1102716eb537d4abf1389af3e938d40
Reviewed-on: https://gerrit.instructure.com/28053
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Paul Hinze <paulh@instructure.com>
Product-Review: Paul Hinze <paulh@instructure.com>
QA-Review: Paul Hinze <paulh@instructure.com>