4.0.0.beta.3: Remove CSS modules, allow dynamic sidebar width, etc.

• Remove mediawiki.skinning.content module per https://github.com/wikimedia/mediawiki/blob/b48c405895a9788b7ba9d9e0ef29f3467015cf98/resources/Resources.php#L88
• Remove some unused Echo modules
• Switch menu sidebar to be hidden with translateY instead of left to allow dynamic size (e.g., a min-width)
• Add new mixin to customize link colors in the toolbox, dropdowns, and categories (and also recolors corresponding SVGs)
• Remove Echo CSS fixes
• Adjust config template to include headers in File namespace
• Use hard tabs consistently per https://www.mediawiki.org/wiki/Manual:Coding_conventions/CSS#Whitespace

Change-Id: Ia893ebcdcb4b2f40ce29c1a9cad9e33b05d391e7