LP#1361270 Self-registration doesn't retain org unit on refresh

LP#1361270 Self-registration doesn't retain org unit on refresh

This sets the self-registration form's org unit selector to the
the current org unit request parameter if it's set and self-
registration is allowed for that org unit. This will preopulate
the org selector on refresh or error instead of resetting to the
default context.

To test:
1. Administration->Local Administration->Library Settings Editor
   set Allow Patron Self-Registration to True (top context)
2. go to OPAC->Request Library Card (stock link is on bottom)
3. select a Home Library and click Submit Registration
4. note the Home Library select doesn't retain your selection
5. apply this fix and repeat steps
6. note the Home Library is retained

Signed-off-by: Dan Briem <dbriem@wlsmail.org>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>