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

@talbs & @adampalay - here are the first three a11y changes from TPG. Let me know if you'd like to see the TPG issues from the report. Sadly a bunch of SASS files were affected to fix one of the issues, thus the 43 files changed...

- Doubled up :hover and :focus styling to improve a11y (this was done in about 40 SASS files...).
- 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.