Vector: New beta module with new typography styles

To enable install Extension:BetaFeatures and Extension:VectorBeta
in LocalSettings.php

Changes to core:
* Refactor screen.less to make use of variables

Introduce dormant skins.vector.beta module
* Activated by https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/VectorBeta
* Load different environment variables depending on whether in a beta
  experiment or not
* Switch global fonts for text from browser default sans-serif to a
  specific sans-serif stack.
* Switch global fonts for headings to Georgia/serif
* Cleanup over-specific specifiers in process
* Adjust styling of navigation menus to focus on content
* Adjust styling of collapsible navigation menu to be less prominent

See https://www.mediawiki.org/wiki/Wikimedia_Foundation_Design/Typography

Change-Id: Ica0b69ad95051e15cd8ed9888927e034c13fbf92
Merge "Vector: New beta module with new typography styles"