Adds 'UPLOADS_ROOT' configuration variable which decouples the 'uploaded/images' location from a being a suffix of MEDIA_ROOT.

This allows the upload directory to be overridden in local_settings.py, allowing multiple instances of Review Board on a system to share a system-wide installation containing the htdocs and media directories (ie. in the context of distributor packages).

For example, HTDOCS_ROOT and MEDIA_ROOT may point somewhere within /var/lib/ or /usr/share/, whilst a particular instance's UPLOAD_ROOT would point to (say) /srv/mysite.tld/uploads/

This change does not affect existing installations.

This diff additionally modifies the "chown" call to use ":" (colon) as the USER:GROUP seperator over the deprecated "." (period).

Tested locally with my (draft) Debian packages.