Update web view to mesh better with iOS 7:

Summary:
- Get rid of some gradients
- Make subtitles highlight extend across entire view
- Update font
- Adjust fonts & button positions on the 'about the video' bar so a little more of the title can show

Test Plan: Load the iPad app pointing to this commit? It looks flatter, and will clash less awkwardly with iOS 7 devices.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3135