[FIX] fields: html: strip_style / strip_classes

Added the support of strip_classes, like the already existing strip_style
parameter of html fields.. This parameter is missing but allow full
customization of html fields.

Also fixed the call to the sanitizer in fields. Indeed the strict mode is
set to False. When not in strict mode the classes are never stripped whatever
the value of strip_classes.

Sanitizer is now called always in strict mode to ensure the sanitize process
is correct and really take into account strip_style and strip_classes.

This fix is done in master due to possible side effects and possible behavior
changes that are not welcomed in stable versions.