Name the cause on the page for a refused form (closes #358)

The CSRF middleware answered a refused login or generator form with the
library's bare text, such as "Forbidden - origin invalid". It now answers
403 with a page that names the cause and what to do, and logs a warning
with the request ID and the library's reason. A page opened over plain
HTTP with debug off and a form from another host both fail the origin
check; the page tells them apart by the host in the Origin or Referer
header.

Model: opus-5-5

README.md

@@ -376,6 +376,9 @@ instead. Plain HTTP is for development on the browser's own machine. The login
 session cookie is always marked `Secure`. Over plain HTTP a browser keeps such a
 cookie only for its own machine (`localhost`), if at all.
 
+A form refused with 403 gets a page that names the cause and what to do about
+it. pixa logs the cause as a warning with the request ID.
+
 The image routes answer the errors listed for them with a JSON body holding
 `error`, `status` and `timestamp`.