Added a "config" core Service

* this allows our code to update the $app['config'] content at runtime if needed
Misc optimizations & bugfixes in phpBb data import process

* SQL queries log sending through HTTP "X-Perfs-*" headers is now disabled for content import batches, since they execute a large number of SQL queries
* a success message is now displayed at the end of the import
* small i18n fixes
* small bugfixes
Misc HTML/CSS enhancements

* added a "Redirecting..." message in our Ajax simple redirect Twig template
* added a missing i18n key in "forum-base" core Plugin
* tiny  visual enhancements in the Twitter Bootstrap test theme
SQL migrations fixes

* some fixes for the "users" table in our SQL migrations
* renamed SQL migrations concatenation script file & output file
Dockerfile update

* Pfwheee! That was hard and long to test, but we finally have a working Dockerfile based on Ubuntu, with a non-root user and auto-started LAMP Services
* Cant' start the MySQL service with "service start mysql", and couldn't resolve this. We have to use "/usr/sbin/mysqld" instead :-/
* The Dockerfile is a little bit ugly, we might use "ADD" Docker instructions later for a cleaner Dockerfile