Allow using markdown in `HtmlCleaningCharField` form fields

[ffont]

Form fields of the type HtmlCleaningCharField (and HtmlCleaningCharFieldWithCenterTag) allow using some HTML tags that are later rendered accordingly. This is used in: * sound comment * forum body * message * sound description * pack description * sound signature * user about text * user forum signature (?)

It would be good to add support for markdown formatting as well as this is quite common nowadays. We already have markdown support for the wiki pages so it should not be too difficult.