Update, remove old design styles in mdn-screen.css

Rules that are still needed are moved to Stylus and updated to work
correctly alongside styles from the new design. Along the way, page
elements and even entire pages are updated to adhere to conventions of
the new design. Spacing is adjusted, colors are updated, responsive
layouts are added, and more.

The design of the Demo Studio is not updated (and Demo Studio rules are
not moved to Stylus) because a project is already planned to completely
redesign these pages. Significant user-facing improvements are made to
the learning pages, but some learning page rules are copied directly
from mdn-screen.css and left untouched. The quality of the underlying
implementation matters very little, in my opinion, as the next major
user-facing change to these pages will likely be removal.

A flag named remove_obsolete_stylesheets is added. When the flag is
enabled, mdn-screen.css is not attached to pages, simulating removal
from the codebase. It may be wise to enable this flag for writers for
some period of time, so that they can test and report on the impact of
this removal before other users are affected.

Note that the flag only controls whether mdn-screen.css is attached to
pages. It does not toggle all changes made here. Updates to page
templates, additions to stylesheets, and other changes take effect even
when the flag is disabled.