Updates a bunch of links to be HTTPS

This adds `https://` wherever possible, and fixes the OpenCongress URL to use `opencongress.org` without a redirect.

I noticed this because [Scout](https://scout.sunlightfoundation.com/) is currently sporting a mixed content warning in Chrome, now that Chrome is flagging secure sites that submit to insecure forms. Fixing the branding bar, and then updating it in Scout, will fix that problem.