Implement a list of "whitelisted" filetypes+extensions for use in templates, etc

[sarasafavi]

At the moment, the "Add file" template includes an outdated list of "Valid file types" (https://github.com/boundlessgeo/django-osgeo-importer/blob/master/osgeo_importer/templates/osgeo_importer/new.html#L52).

As we add support for additional file types it might make sense to pull a dynamically-generated list into the template instead of hardcoding in the markup.

[sarasafavi]

This has been resolved with 39522d2 (within #20)