Hg 124 (Extract critical, "above the fold" CSS and place it inline)

Summary of the changes:
* Extracting "above the fold" SCSS to preload/ directory, which is included in the style tag in the head of the html document through baseline.scss.

* Changing rem-calcs to rem where needed, since we wanted to get rid of dependencies in SCSS to keep inline css as small as possible.

* Loading seperate stylesheet (app.css) using defer rather than rendering it blockingly (as we did before) or lazy-loading - defer makes it render in non-blocking way just before DOMContentLoaded, which helps in getting rid of weird transition effects and flickering inline SVGs while removing .preload class.

* Rendering article title only for existing articles (if we'd keep rendering in a way we did, we would visually change bad article title from the request (e.g. "Rachel Berry2") into "Error XYZ" on the front-end side. Right the title comes empty and let's front-end code generate the error title.

* There is a significant boost when we take a look at the first time user can see the article on the screen. I will run the tests again and report them on JIRA.

ping @hakubo @kenkouot @rogatty @mklucsarits 
This screams that it can't find article