propery re-enable disabled elements w/o opacity set. fixes #10225
propery re-enable disabled elements w/o opacity set. fixes #10225

when an element didn't have an explicit opacity set,
disableWhileLoading would leave the opacity at 0.5 in
IE.

this commit fixes the problem by assuming that, if no opacity
is set, it should be 1.

test plan:
  * create a course with students;
  * visit the users page of that course (/courses/:id/users)
    in IE8;
  * verify that the student and teacher lists load and are
    displayed as fully opaque.

Change-Id: Iad16cfbe01cf841209d1ca6ddb99b86eabd69d75
Reviewed-on: https://gerrit.instructure.com/13287
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Zach Wily <9de8c4480303b5335cd2a33eefe814615ba3612a@instructure.com>

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