Added a new Plugins Manager "ActionsVariablesConvertersManager" Behaviour

* with this new Plugins Manager Behaviour, we can now use Silex "routes variables converters"!
* we now have a new public method on the ActionsManager (available through the PluginsManager, like all Behaviours methods), "runActionFile()". This will allow us to run other Actions from an Action or a Middleware
Better "authentication required" behaviour

* we an anonymous User tries to access a "authentication required" resource, we now display the "sign in" form without redirection, with a proper 403 HTTP status code
* we also display a dedicated alert message when this happens
First successful "new Post" creation. Yeah! Misc optimizations & bugfixes

* authenticated Users can now create new Posts
* they have a "wysibb" WYSWYG BBCode Editor to help them
* added our first 2 Actions variables convertes, "forum-id" and "topic-id"
* added somes helpers on our Topic Model
* new "bbcode_to_html" Twig filter: this is much cleaner than our previous Hooks-based solution for BBCode parsing
* Ajax alerts display optimizations & bugfix
* SettingsManager bugfix
* TWB test theme: misc visual tiny enhancements
* Bower: added "jqjquery-wysibb" library & RequireJS "css!" loader
* added missing Bower install to the Dockerfile
* misc enhancements & bugfixes here and there