MIT: keep experiment data in the session rather than query parameters

Generating a user key on each view of the county performance page, and
passing it on as a parameter, means that if the same user returns to
the page later (e.g. by pressing "Back" after a sentate comment
submission) they'll probably have a different user key - this is
problematic, and we can easily get around it by storing the user key,
variant and facebook demographics in the session.
MIT: Add some docstrings and comments
MIT: Make the share / survey redirects temporary rather than permanent

This is so that the browser won't cache the redirect so aggressively.
MIT: Add tests for the share links and the survey link
MIT: Make the survey button make a POST request
MIT: stop web crawlers from exploring /county-performance