Extend Django detection

Sadly Django sites are hard to detect, as they have a policy of not leaking information.
However, there are two ways to detect Django:
* At pages with HTML `<form method="post">` tags, Django developers typically include the `csrfmiddlewaretoken` field in the form.
* In the admin site, there is a `window.__admin_media_prefix__` in the HTML.

This patch adds detection for both features.

Two remarks on the csrfmiddlewaretoken:
* The indication may not necessarily be present at all pages of the site.
* The attribute order in `apps.json` is based on Firefox's output of the `innerHTML` code. The real source code uses single quotes and a different ordering of attributes.

Nevertheless I hope you find this enhancement useful and accept the patch.