Round 2 Accessibility Changes

@talbs, @frrrances and @markchang 

This round does the following:

- uses jQuery to focus on alert messages when they are shown to help both keyboard navigation and mouse users.
- makes course sequence titles available to screen readers.
- removes ::selection styling from CSS and lets browsers use their defaults.
- adds ALT tags to course cover images both in case the image doesn't load (or loads slowly) and to clue in screen readers
- increases contrast of lightest text and darkens background behind institution flag on course cover images
- makes focus state very deliberate and obvious to help keyboard navigation users