Minimize repetition in the variables passed to `render`

[davemfish]

All model templates should be rendered with these variables, in addition to other model-specific variables:

We can try to minimize the need to define all of these in every reporter.

    with open(target_html_filepath, 'w', encoding='utf-8') as target_file:
        target_file.write(TEMPLATE.render(
            report_script=__file__,
            timestamp=time.strftime('%Y-%m-%d %H:%M'),
            model_id=model_spec.model_id,
            model_name=model_name,
            model_description=model_description,
            userguide_page=model_spec.userguide,
            args_dict=args_dict,
            model_spec_outputs=model_spec.outputs,
            ...
        ))

• Perhaps we can define a wrapper/decorator that deals with defining the report_script and timestamp.
• page_title can probably be removed entirely and constructed within base.html.
• We could consider passing in the entire model_spec instead of various attributes, but I'm not sure if that's a good idea. It offloads the attribute access to the templates.