Fix missing template vars errors

[michaeljcollinsuk]

• Add conditional to check for required_css_class attr on a form
• Use 'static' rather than 'staticfiles' templatetag to resolve removed in django 3 deprecation warnings
• Bump minor version to 0.3.13

Co-authored-by: name virginia.dooley@democracyclub.org.uk Co-authored-by: name michaeljcollinsuk@users.noreply.github.com

[michaeljcollinsuk]

@symroe after merging, is there an additional step to take to publish the new version?