support <track> elements in rich content

fixes CNVS-30404

test plan:
- add a video/audio with a track tag in the html editor
- toggle back and forth between rich text and html and make sure it
stays
- save and edit and make sure it stays

Change-Id: I18418ef98ed2c03d1a4901a4bf15cf82474c6131
Reviewed-on: https://gerrit.instructure.com/85159
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>

gems/canvas_sanitize/lib/canvas_sanitize/canvas_sanitize.rb

@@ -80,7 +80,7 @@ module CanvasSanitize #:nodoc:
           'del', 'ins', 'iframe', 'font',
           'colgroup', 'dd', 'div', 'dl', 'dt', 'em', 'figure', 'figcaption', 'i', 'img', 'li', 'ol', 'p', 'pre',
           'q', 'small', 'source', 'span', 'strike', 'strong', 'sub', 'sup', 'table', 'tbody', 'td',
-          'tfoot', 'th', 'thead', 'tr', 'u', 'ul', 'object', 'embed', 'param', 'video', 'audio',
+          'tfoot', 'th', 'thead', 'tr', 'u', 'ul', 'object', 'embed', 'param', 'video', 'track', 'audio',
           # MathML
           'annotation', 'annotation-xml', 'maction', 'maligngroup', 'malignmark', 'math',
           'menclose', 'merror', 'mfenced', 'mfrac', 'mglyph', 'mi', 'mlabeledtr', 'mlongdiv',
@@ -156,6 +156,7 @@ module CanvasSanitize #:nodoc:
           'embed' => ['name', 'src', 'type', 'allowfullscreen', 'pluginspage', 'wmode',
                       'allowscriptaccess', 'width', 'height'].freeze,
           'video' => ['name', 'src', 'allowfullscreen', 'muted', 'poster', 'width', 'height', 'controls'].freeze,
+          'track' => ['default', 'kind', 'label', 'src', 'srclang'].freeze,
           'audio' => ['name', 'src', 'muted'].freeze,
           'font' => ['face', 'color', 'size'].freeze,
           # MathML

gems/canvas_sanitize/spec/canvas_sanitize/canvas_sanitize_spec.rb

@@ -33,4 +33,9 @@ describe CanvasSanitize do
     cleaned = Sanitize.clean("<p data-item-id='1234'>Item1234</p>", CanvasSanitize::SANITIZE)
     expect(cleaned).to eq("<p data-item-id=\"1234\">Item1234</p>")
   end
+
+  it "does not strip track elements" do
+    cleaned = Sanitize.clean("<track src=\"http://google.com\"></track>", CanvasSanitize::SANITIZE)
+    expect(cleaned).to eq("<track src=\"http://google.com\"></track>")
+  end
 end