[issue2551231] template.py-HTMLClass::classhelp doesn't merge user defined classes

New submission from John Rouillard:

if classhelp is called with html_args={"class": "some_class"}, the output
has two class attributes which is invalid html.

The returned string is generated by:

        return '<a class="classhelp" data-helpurl="%s" data-width="%s" data-height="%s" 
href="%s" onclick="%s" %s>%s</a>' % 
               (help_url, width, height,
                help_url, onclick, self.cgi_escape_attrs(**html_kwargs),
                self._(label))

which hard codes a class="classhelp". Better way to handle this is to
check html_kwargs for 'class' key and append the value (with a space)
to a new class_value variable (initialized to "classhelp"), then remove
the class element from html_kwargs.

Replace the hard coded class value for the new class_value variable.

----------
components: Web interface
keywords: Effort-Low, StarterTicket
messages: 7642
nosy: rouilj
priority: normal
severity: normal
status: new
title: template.py-HTMLClass::classhelp doesn't merge user defined classes
type: behavior
versions: devel

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