Change template context generation in TemplateHTMLRenderer

[rfleschenberg]

Make TemplateHTMLRenderer more subclass-friendly. Refs #4230.

• Change the name of resolve_context() to get_template_context().
• Pass the renderer context to this method, to give subclasses more flexibility
  when overriding.

This allows users to write code like this:

class ViewInContextHTMLRenderer(TemplateHTMLRenderer):

    def get_template_context(self, data, renderer_context):
        context = super().get_template_context(data, renderer_context)
        context['view'] = renderer_context['view']
        return context

Or maybe even:

class ExtraContextHTMLRenderer(TemplateHTMLRenderer):

    def get_template_context(self, data, renderer_context):
        context = super().get_template_context(data, renderer_context)
        context.update(renderer_context['extra_template_context'])
        return context

Currently, you have to override and reimplement the whole render() method instead.

This change does break compatibility with existing code that touches this interface. As far as I see, the interface was not documented. If desirable, we could add a compatibility shim.

[tomchristie]

Broadly in favour, yup.

[tomchristie]

As far as I see, the interface was not documented.

Going to merge this, given the above.