/dashboard/ URL is hardcoded

Hi, thanks for this amazing Django app!

https://github.com/simonw/django-sql-dashboard/blob/cc1dfd30494af8cb95b35aacc7f60b18c11c6f4b/django_sql_dashboard/templates/django_sql_dashboard/saved_dashboard.html#L14

I think this should use `{% url 'django_sql_dashboard-index' %}` instead of hardcoding `/dashboard/`