A11Y changes: better :focus styling, increased contrast, and added image alt texts

- Doubled up :hover and :focus styling to improve a11y
- Increase contrast of certain UI elements for improved a11y
- Added some image alt text for a11y
- Changed video caption styling to blue and made them underline on hover and added a skip link before video for screenreaders.

Fixes Bugs:
- LMS-1336
Merge pull request #1479 from edx/giulio/a11y-batch1

A11Y changes: better :focus styling, increased contrast, and added image alt texts