Bug 8203: Add ability to save individual authorities

Bug 8203: Add ability to save individual authorities

Adds the ability to save individual authority records in MADS, MARCXML, or
binary MARC format to the staff client.

To test:
1. Apply patch
2. View authority record in staff client
3. Try saving record as MADS, MARCXML, and MARC, and confirm that the
   resulting files are what you expect

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works nicely, tested different export options on different records.