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

internal/templates/form_refused.html

@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Pixa - Form Refused</title>
+    <link rel="stylesheet" href="/static/style.css">
+</head>
+<body class="login">
+    <div class="card">
+        <h1>Form Refused</h1>
+
+        <div class="error">
+            {{.Cause}}
+        </div>
+
+        <p>
+            <a href="/">Back to the start page</a>
+        </p>
+    </div>
+</body>
+</html>