Limit width of text/enriched

60em is a bit more than 100 characters in the fixed-width font I use so
that should be an acceptable compromise between readability and avoiding
unintended line breaks.

The RFC says that output should use "the maximum available margins", but
that was clearly before large monitors became common.

style/debug.css

@@ -28,6 +28,7 @@ th {
 .text-enriched {
     font-family: monospace;
     white-space: pre-wrap;
+    max-width: 60em;
 }
 
 .signature {