Merge.

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branch : per_site
Optionally allow the media library to be segregated on a per site basis, configurable via a Mezzanien editable setting.

--HG--
branch : per_site
Replaced a few more instances of DIRECTORY.  Cleaned up comments.

--HG--
branch : per_site
Merged in joshcartme/filebrowser-safe/per_site (pull request #6)

Media library segragation based on site
Fix dir paths for MEDIA_LIBRARY_PER_SITE.
Move root dir creation to new get_directory().
Move MEDIA_LIBRARY_PER_SITE to Mezzanine so it will be documented.
Don't assume Mezzanine has MEDIA_LIBRARY_PER_SITE yet.