fix media comment url not showing

closes EVAL-2327
flag=none

Test Plan:
1. Go to SpeedGrader and create a media comment.
2. Click to play the media comment, then click on the "CC" button and
   then click "Upload subtitles". In the modal that shows up, verify
   in the Step 1 section there is an input with a URL in it.
3. Click the "x" in the top-right corner of the modal. Then, click "CC"
   and "Upload subtitles" again. In the modal that shows up, verify
   in the Step 1 section there is an input with a URL in it.

Change-Id: Ia1fbe8c84adf4496ba2be5beb94ac48d9cac0dcc
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/293626
Tested-by: Service Cloud Jenkins <[email protected]>
Reviewed-by: Aaron Shafovaloff <[email protected]>
Reviewed-by: Eduardo Escobar <[email protected]>
QA-Review: Aaron Shafovaloff <[email protected]>
Product-Review: Jody Sailor

ui/shared/mediaelement/UploadMediaTrackForm.js

@@ -40,6 +40,7 @@ export default class UploadMediaTrackForm {
     this.$dialog = $(template(templateVars))
       .appendTo('body')
       .dialog({
+        close: () => this.$dialog.remove(),
         width: 650,
         resizable: false,
         buttons: [