Adds labels and ARIA hints for screen-readers

This is to try to help screen readers understand better the structure of the data under "Recently Uploaded" and "Featured Works" on the home page. 

The Mozilla web site recommends tagging headers and content with role="tab" and role="tabpanel" to that the screen readers can provide more context ( https://developer.mozilla.org/en-US/docs/Web/Accessibility/An_overview_of_accessible_web_applications_and_widgets) I also added a few screen-reader only labels. 
@jcoyne good point, I've replaced them with spans