This change calls for an explanation:
- `RenderMixin` doesn't seem to be adding anything here, so it is
gone.
- The web browser was unhappy without a charset in the response
(Firefox 74 was anyway), so `content-type` header also gets a
`charset=UTF-8`.
- Returning a Unicode string made nevow appserver unhappy, so it is
just a `str`. The precise error message was:
exceptions.TypeError: ('Could not adapt', u'An incident report has been saved to logs/incidents/ in the node directory.', <InterfaceClass nevow.inevow.IResource>)
Fixes: ticket:3294
