Access standard metadata formats in public interface

This adds two configuration options:

1. Option to allow "raw" metadata to be retrieved in
the public interface for resources (ead, marcxml) and
digital objects (dc, mets, mods) by appending /:format
to the url, which helps to address:

https://groups.google.com/forum/#!searchin/archivesspace/oclc/archivesspace/fwjfMI7IGjQ/nYdHm4SvdBEJ

It also provides a convenient, more immediate way to
view how ArchivesSpace converts objects into those
standard formats without exposing the backend directly.

2. Option to enable links to the "raw" metadata urls in
the navigation sidebar.