Bug 26916: Show searchable patron attributes in patron search dropdown

Bug 26916: Show searchable patron attributes in patron search dropdown

This patch adds attributes that have been marked searchable to the patron
search dropdowns

To test:
1 - Define some new patron attribute types and make then staff_searchable
2 - Add some values to patrons
3 - Confirm they are searched in 'Standard' search, but there is no other way to search them
4 - Apply patch
5 - Confirm the fields now show in patorn search dropdowns
6 - Search using 'standard' and confirm the searhc works
7 - Search specific attributes and confirm the searhc works

Signed-off-by: Janusz Kaczmarek <januszop@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>