Improve performance in views integration

Rigth now, a typical casetracker view with 20 rows is calling to check_plain 60 times.

This is because check_plain is placed into render() functions of casetracker field handlers.

However, this can be improved in a simple way. Since case realms are limited data, we can get they in the constructor of each feild handler sanitize this string here.

This reduce from 60 to 10 aprox. the calls to check_plain() in a casetracker view.