luci-base: luci.js: add ability to add "preload" classes

Extend the LuCI bootstrap procedure to scan for class files in
/www/luci-static/preload/. Any JavaScript file found there will be
required automatically before setting up the view, allowing to stage
code that should run on every page load.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>