Allow searching for *any* priority
forms: Add placeholder and promts

Priority field and custom list fields have a prompt value (shows up only for
drop-down widget). And the short and long answer fields have a placeholder
configuration item.
Add placeholder for typeahead lists
Merge pull request #547 from greezybacon/issue/search-no-priority

forms: Allow searching any priority

Reviewed-By: Peter Rotich <peter@osticket.com>