Missing tab structure for media gallery tabs

[joe-replin]

Bug Description:

Media gallery tabs do not convey the following information:

Accurate labels that include the visual text of the controls
tab structure to indicate the expected behavior of the controls
The selected tab within the grouping 

Steps to Reproduce:

Using a screen reader, open the course prototype.
Open the standard experience, then locate the Media gallery section
Navigate through the 3 video tabs in the gallery and note the description of the elements. 

Actual Behavior:

See bug description.

Expected Behavior:

Refer to the ARIA Authoring Practices for guidance building accessible tab elements: [​https://www.w3.org/TR/wai-aria-practices-1.2/#tabpanel](https://www.w3.org/TR/wai-aria-practices-1.2/#tabpanel)
Ensure the labels of the tabs include the text of the control's visual label (ex: Video 1, Subtitle)