Remove nevow from allmydata.web.root.IncidentReporter

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

src/allmydata/web/root.py

@@ -28,7 +28,7 @@ from allmydata.web.common import (
     WebError,
     get_arg,
     MultiFormatPage,
-    RenderMixin,
+    MultiFormatResource,
     get_format,
     get_mutable_type,
     render_time_delta,
@@ -169,14 +169,15 @@ class FileHandler(rend.Page):
         raise WebError("/file must be followed by a file-cap and a name",
                        http.NOT_FOUND)
 
-class IncidentReporter(RenderMixin, rend.Page):
-    def render_POST(self, ctx):
-        req = IRequest(ctx)
+class IncidentReporter(MultiFormatResource):
+    """Handler for /report_incident POST request"""
+
+    def render(self, req):
         log.msg(format="User reports incident through web page: %(details)s",
                 details=get_arg(req, "details", ""),
                 level=log.WEIRD, umid="LkD9Pw")
-        req.setHeader("content-type", "text/plain")
-        return "An incident report has been saved to logs/incidents/ in the node directory."
+        req.setHeader("content-type", "text/plain; charset=UTF-8")
+        return b"An incident report has been saved to logs/incidents/ in the node directory."
 
 SPACE = u"\u00A0"*2