This fixes a weird issue where the following sequences of tests would fail:
test-backend
zerver.tests.test_messages.PersonalMessagesTest.test_personal_to_self
zerver.tests.test_report.TestReport.test_report_error
zerver.tests.test_templates.TemplateTestCase.test_custom_tos_template
It appears that all 3 tests are required for the failure.
While it's not entirely clear what the cause is, a very likely factor
is that settings.DEBUG is special, and so changing it at runtime is
likely to cause weird problems like this.
We fix this by replacing it with settings.DEVELOPMENT, which has the
same value in all environments, but doesn't have this problem of being
a special Django thing.