Improve subtitle loading and error message

Summary: {F11324}

Reviewers: laura

Reviewed By: laura

Differential Revision: http://phabricator.khanacademy.org/D6309
Update share and download icons

Summary:
I was going to use Font Awesome directly but this turned out to be easier.

{F11332}

Reviewers: laura

Reviewed By: laura

Differential Revision: http://phabricator.khanacademy.org/D6313