Adds sticky loader when transitioning pages.

Resolves #272 

Todo:

- [x] Figure out why the loader moves over a few pixels on Chrome
- [ ] Browser test

Answer to 1:

It likely moves over because the body's min-width no longer pushes it those few pixels over. When the browser window is < the size of Gistbook's width, it will do the shift. Removing the min-width fixes the problem.