Revert back to "django" as the default setting for "context_behavior"

[JuroOravec]

See the thread in #494

TL;DR:

#494 (comment)

I wonder if we should switch back to "django" as the default context_behavior, and have isolated be a mode the we reserve for everyone that want the kind of detailed scope control you are after? I think that might be the best path here, that both gives you the control you want, but does not complicate things for beginners?

and #494 (comment)

I think we should [...] be very specific when in isolated mode, but at the same time switch back to "django" as the default. I fear that the kinds of issues you bring up are for later in a project lifetime, are great for maintainability, but might hinder some people that are getting started and don't (yet) need strict isolation. They can always add it later on.

Notes:

• Ideally we'd update tests so that those tests that render templates would be would be duplicated to be tested for both the "djagno" and "isolated" settings.