Move custom field "Required" setting to ideas portal form layouts

The same custom field may be required in some places (e.g. in some idea portals), but not in others. Customers work around this now by creating two copies of the custom field, one that is required and one that is not.

The required flag should be a setting of the layout so that each layout can customize this setting.

Note that the "Required" flag only applies to idea portal layouts.