Propagate current tab URL from add-on to extension front-end

 * When the extension's panel is loaded for the first time,
   notify the front-end of the active URL

 * Share webui/page_access.ts module which defines the API for
   interaction between the browser-specific extension and
   the front-end between the front-end and the extension code.

   The content script for the extension's panel is now browserify-ed
   so that it can use shared interfaces from webui/

 * Disable strict mode when initialization the extension to suppress
   a large number of warnings when visiting common websites which
   hide logging messages from the app itself.

   This preference modification should in future only be applied
   when developing the extension.