Added site base URL configuration parameter.

Added a site base URL configuration parameter (to both the
settings page and the purge task).
Added URI identifier to MODS export.
Changed order of MODS export repository fields.
Changed MODS bulk export so children export.

Changed the query for information object so if an export's format is
MODS, and the current level only option isn't flagged, child
descriptions are exported in addition to top-level descriptions.
Removed typo in MOD XML template.
Added XML tidying to bulk export.