WIP on current JS integration with Twitter Flight

* our current JS components are progressively replaced with Flight Components
* we can now add multiple Components on a single DOM node
* added a "$app['html-components.add_component']()" function to easily add a JS Flight Component on a DOM node
still WIP on current JS integration with Twitter Flight

* our current JS components are progressively replaced with Flight Components
* added a Flight "Ajax forms" Component
* added a "withAlertsCapabilities" Flight Mixin
* our Alerts display now clear itself on "mainContentUpdate"
* bugfix for Ajax breadcrumbs on our test TWB theme
still WIP on current JS integration with Twitter Flight

* added a single entry point for our miscellaneous "Ajax navigation" Flight Components
* JS files CS fixes (we use double quotes everywhere instead of single quotes)
still WIP on current JS integration with Twitter Flight

* we have almost clean Flight Components distribution for our Ajax navigation :-)
* added a simplistic "debug" flag to allow "minified or not" loading of thir-party JS libraries
Getting better and better on JS Flight migration

* the AJax layout has been moved to the "ajax-navigation" Core Plugin, and renamed from "ajax.twig" to more explicit "ajax-layout.twig"
* our Flight Components are getting better and better. Twitter Flight, I ♥ you :-)
Added some advanced and real-time app performances metrics

* we now display Ajax loadings stats & app performances metrics
* added a bunch of "X-Perfs-*" HTTP headers
* this can be enabled and disabled with the config flag "debug.perfs-tracking"
* added "scroll to top" on Ajax content update
* misc optimizations & bugfixes
Merge branch 'javascript-refactoring-to-twitter-flight'