Migrate mdn-screen.css to Stylus

Some groundwork is included here. To summarize, the css_migration flag
is added to enable the effects of the migration. When the flag is
enabled, a modified version of mdn-screen.css (with obsolete and
migrated rules removed) is attached to pages. When the flag is disabled,
the normal mdn-screen.css stylesheet is attached instead.

The flag should be disabled for almost all users, but it might make
sense to enable the flag for beta testers so that they can find bugs in
the migration and report on them.

The cost is one additional request. Worth it for an implementation that
is simple and understandable, in my opinion.
Remove obsolete blockquote, search, .hidden styles
Restore some needed button properties
Remove rules for #logo, #site-search, footer, more
Put .button properties above mixins
Override text-decoration instead of using :not()
Remove .aside and #branding
Migrate input rules, remove layout/header rules
Migrate .notice rules
Migrate ol.prose
Remove lots of unused rules
Remove some additional rules
Remove rules, image formerly used on /newsletter
Move some demo-only rules to demos.css

Two things to note here. First, the rules are moved to the old demos.css
stylesheet intentionally, as these rules will probably become irrelevant
once we redesign the Demo Studio. Second, I remove the rules from
mdn-screen.css in addition to mdn-screen-migration.css so that people
not testing the migration do not have the rules applied twice.
Remove ul.index rules
Remove "HTML5 & Friends" rules (see #1938)