Demo day video/link and linking policy changes

This pull request does several things to address issues #40 and #43:

* Remove linkedin links from the team section (github links left in per @quepol)
* Add video to the right of the mantra (and narrow the mantra itself to make room)
* Add button to go sign up at eventbrite
* (Temporarily) remove the lightbulb background image from behind the video
* Add the linking-policy.html page
* Add a column to the footer for policies and link to the linking policy
* Remove the modal popup for people leaving the site.