More changes from our call yesterday

This is changes from our call yesterday: text fixes, overlap fixes on other pages, new nav and login, and fixed position header (which takes the most css).

The CSS here might look like more than needed like the media queries, but each one comes from experimenting with various sizes and scroll positions and I think it is important to get it right since otherwise stuff was overlapping with the wrong kind of text. I'm open to changing if there's better bootstrap stuff but I like this.
Ditto on above comment about Bootstrap's media query support.  I don't think we want to hard-code the pixels for these media queries because they are dependent on the Bootstrap behavior which could change.