Use Django admin base_site templates

[JobDoesburg]

Removes a bootstrap dependency and uses the Django admin base_site template for IdP test and error pages

[dcwatson]

Thanks for the PR, I like the idea! However, while it does remove the bootstrap dependency, it also means you need to have django.contrib.admin installed to find the base templates. In reality, these templates should be overridden for production sites anyway, but I think having a default that works without the admin installed is probably a good thing.

I'll leave this open to think on for a little while. Let me know if you have any more thoughts about it.