Fix template insertion in twig.form.resources

We can't be sure that the core template will be in first position (see https://github.com/symfony-cmf/RoutingExtraBundle/pull/48/files#r2514138)

Now the core template is searched and if it exists, mopa's template is inserted right after.
Merge pull request #356 from EmmanuelVella/patch-1

Fix template insertion in twig.form.resources