A number of important accessibility changes

@talbs, @frrrances - here's the PR for the accessibility items...currently assigned to @talbs, but can be re-assigned

- added aria-required="true" attributes to required form fields
- added hidden labels to user button and drop down
- added active state label to course tabs
- added active state label to courseware accordion chapter and subsection tabs/links
- reordered courseware subsection sequence navigation so that previous arrow appears before sequence items