Incremental fix for Bug 3285 - Location on Add Subscription should be pull down

Incremental fix for Bug 3285 - Location on Add Subscription should be pull down

- Adds location dropdown to subscription edit form
- Pulls authorised value for display on subscription detail page
- Adds function for pulling authorised value description based
  on category and value

This patch does NOT implement automatic preselection of the shelving
location form field on the serial add item screen. This must be
worked out in order for the bug to be closed.

Signed-off-by: Colin Campbell <colin.campbell@ptfs-europe.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>