Add a method to specify class on form help text.

[James Tocknell]

Currently, the help text on a form generated with the standard helpers always uses "helptext" as the css class on the help text, with no current way to override this (other than creating the form manually). It would be useful to have a setting (e.g. FORM_HELP_TEXT_CSS_CLASS) to change the class, or to be able to override/set the class via some other method. I'm happy to create a PR with the new setting, if that's acceptable, or create a patch to solve this some other way.

[Mariusz Felisiak]

Thanks for this ticket, however creating a new setting is always controversial and we don't want settings for such small customization. Moreover creating a new form is not complicated so it doesn't justified adding a new option for this. You can always customize helptext style for a specific form ID.