Update Event data fixtures to load fixture posters

The LoadEventData fixture class now duplicates the fixture poster files
to the right directory automatically.
Make getDirectory() methods static methods

They can now be called on the class itself and not only on instances of
the class.
Rename @button-width LESS variable to avoid collisions

Rename @button-width to @event-show-event-button-width to avoid almost
certain collision.
Remove useless parenthesis in custom-other.less

There is no need for parenthesis using LESS mathematically computed
values.
Indent and space correctly global LESS files

Two blank lines before sections.
Use "getPosterPath()" method to display event poster

In event page, use the "Event::getPosterPath()" method to get
automatically the absolute path of the poster image file.
Refactor the Twig Extensions files

Add a class header and method headers for each of them, and uses the
"filter_function" and "test function" when using a PHP function.