Enable Django Debug Toolbar in development

[davidfischer]

In the past few days I have found a few performance issues that could have been caught easier with Django Debug Toolbar (#5451 and #5460). I propose we enable it in development.

[stsewd]

I think we should put this in another file like https://github.com/stsewd/readthedocs.org/blob/b856e791b6be6cff68a98efd1422f30136c2d856/requirements/testing.txt#L27-L29? Mostly because this will be installed in production too, we are not going to activate it, but looks like a dead package to have.

[davidfischer]

The reason I put it here is that it is required for testing, development, and linting. Currently only the default requirements.txt file (which then loads pip.txt) is used in development according to our documentation.

[stsewd]

My comment can be addressed in another PR if considered, I'll like to try this now :)

[davidfischer]

I'm going to merge this now, but I think @stsewd's comment is a fair one. Overall, I think we need to be very clear what each of our requirements files are for. We also might have 1-2 too many requirements files (maybe just dev, test, and prod?). These are issues for another PR though.