Use Symfony property accessor in twig extension

Uses the PropertyAccessor to correctly determine the getter method for non-trivial property access.

A camel-case private property on an entity (e.g. myProperty) should be used in a form with underscore naming convention (e.g. my_property). Determining the getter method using `'get'.ucfirst($prop)` will fail as the method `getMy_property()` will not exist on the entity.