Refactor `application.css.scss'.

Previously, much CSS was located in `application.css.scss'. This wasn't
good, since 1) much of the code was page specific, and 2)
`application.css.scss' shouldn't contain styling itself. So, I've
removed the styling, refactored it into different files, and instead
imported the site-wide ones inside `application.css.scss'. For the page
specific ones, I've included them manually inside each page.