Roughed in AIP recovery functionality.

* Added recovery logic to Package model
* Added REST endpoint to submit recovery requests to
* Added AIP recovery request administration to UI
Added corrupt files backup during AIP restore.
Added auto-creation of recovery location.

Got rid of the recover_backup_path_within_location setting and added
auto-creation of the recover and recover/backup paths within the
internal storage space.
Error if restore attempt woth no files.

Added error notification if trying to restore an AIP without the
necessary files present in the restore directory.
Replaced use of identifier with constant.

Replaced use of an identifier string with a constant.
Prevent creation of redundant recover locations.

Added logic to prevent the creation of more than one recover location per
pipeline.
Add request event ID to event request responses.

For the REST endpoints used to create request events
(AIP deletion request, AIP recovery request), return the
ID of the events event in the JSON returned upon success.
AIP request optional remote endpoint notify.

Added configuration options/logic to enable remote REST
endpoints to be notified upon approval/rejection/failure of a
requested AIP operation. Enabled for AIP recovery.
Add Sphinx doc of AIP recovery admin/process.