Templates broken after django-web-themes added a light/404.html

django-web-themes recently got a light/404.html which is found by our django whenever a 404 is raised (as we have /path/to/web-themes/light/ on the list of template_dirs).

The problem is that light/404.html extends light/index.1col.html which is not found and so our tests fail.

I chatted with mhall119 about it, and the intended way of using the light web theme is for us to use "/path/to/web-themes/" in the template dirs, and then our own 404.html can simply inherit from the light/404.html one.

For us to do this requires a tiny patch to light/404.html so that we can include the extra oops info.

Note: When this fix is going to be deployed, we need to
1) update our configs on both projects with the new config schema option for preflight_base_template (see MPs)
2) Update the template_dirs on the configs (removing the 'light/')
3) Update the templates/404.html and /500.html in the rnr configs (in the same way as the django_project/templates the rnr MP below - maybe we should just move them on trunk into src/sca/templates so we don't need the duplication)