Add chosen support to HTMLForm's multiselect field

For very large multiselects, the list of checkboxes is
overwhelming. This change adds support for using jquery.chosen
conditionally on multiselect fields, if 'chosen' is used for the
cssclass parameter.

Change-Id: If1eba7fb705089deb4cff7a326db0ac803a522ec
Merge "Add chosen support to HTMLForm's multiselect field"