feat(panettone): don't use _ for em in inline markdown

Since we are still using third_party, underscores are kind of common in
issue titles and are probably often forgotten to escape. Let's just
support `*` for emphasized text in titles for now.

Change-Id: I305bcf4d4c59123bba4ce816a6da2ee8b022c34e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2926
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: glittershark <grfn@gws.fyi>

web/panettone/src/inline-markdown.lisp

@@ -3,7 +3,6 @@
 
 (define-constant +inline-markup-types+
   '(("~~" :del)
-    ("_"  :em)
     ("*"  :em)
     ("`"  :code))
   :test #'equal)