Rollback Django Debug Toolbar from 1.0.1 to 0.9.4

We recently tried to update our version of Django Debug Toolbar from 0.9.4 to 1.0.1 (#2391). Doing so caused a lot of problems -- at first, being unable to run the codebase at all due to backwards-incompatible changes in the toolbar's Python code, and then when that was resolved, broken Javascript due to the interaction between DDT's Javascript and ours. Since [DDT is still figuring out how to move forward with their Javascript changes](https://github.com/django-debug-toolbar/django-debug-toolbar/issues/535), we're going to rollback to 0.9.4 for now. I'm also adding a note to the codebase explaining the situation, so we don't run into this again, and once again get bitten by insufficient testing.

@cpennington @cahrens @yarko Can you verify that this is what we want?

Sorry for the instability in the project, everyone. :sweat: