Add Django tests

[jpmckinney]

At minimum "the page loads without error".

Can use https://docs.djangoproject.com/en/3.2/howto/initial-data/#initial-data-via-fixtures like in https://github.com/open-contracting/covid-19-procurement-explorer-admin

[jpmckinney]

Update: Most of the controller project is tested. Once #77 is done, the remaining views in the api project should be ported over. That will leave the exporter project, but we might not want to test it yet (#112).