Rollup merge of rust-lang#60383 - GuillaumeGomez:fix-position-source-code-files-toggle, r=Manishearth

Fix position source code files toggle

Fixes rust-lang#60381.

The second commit is a big cleanup of the media queries.

r? @rust-lang/rustdoc

cc @Manishearth

screenshot of the fix:

<img width="501" alt="Screenshot 2019-04-29 at 23 42 56" src="https://user-images.githubusercontent.com/3050060/56929111-112b2b00-6ad9-11e9-9a23-e0a8e3641395.png">

Author: Centril