Fix django-debug-toolbar by reducing worker count

[agjohnson]

Debug toolbar has been pretty useless for a long time now, all pages in
the toolbar mostly show failures and only work a small portion of the
time. Reducing the worker count resolves this behavior and allows all
pages to function.

There might be a longer term fix here, the problem is likely the workers
not sharing some state that debug toolbar is expecting.

[stsewd]

I think it was set to multiple workers to have something more similar to production to catch errors more early. We could have a debug mode that enables DDT and sets the number of workers to 1 too, but I'm fine with this too.

[agjohnson]

Yeah, though if we special case it, we likely won't use it. There might be a proper fix at the debug toolbar eventually too. But I'm fine getting back to that later and fixing the toolbar for now.

I think the toolbar should be quite important for development on the templates. It seemed like a struggle to debug templates without the toolbar, I'd like to make sure we're all using a workflow that actually works. The toolbar is the best way to inspect what templates are used and the context data to them, it's just unfortunate that it doesn't work with multiple workers currently.