feat(panettone): render a subset of markdown in issue subjects

This is achieved by implementing a simple markdown renderer in CL which
has the following limitations:

* Only supports inline `code`, *emphasize 1*, _emphasize 2_ and
  ~~strikethrough~~.
* Does not support nested markup.

This allows for a relatively simple renderer which doesn't need to parse
markdown into a in-memory data structure first. The rendered result is
directly written to a stream to integrate well with cl-who which is also
reused for rendering tags and xml-escaping strings.

Fixes #90.

Change-Id: Ice88ed770b1fab6365f3b93e8663e25077befa0b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2389
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: tazjin <mail@tazj.in>

web/panettone/src/packages.lisp

@@ -7,6 +7,11 @@
   (:use :cl :lass)
   (:export :styles))
 
+(defpackage panettone.inline-markdown
+  (:use :cl)
+  (:import-from :alexandria :define-constant)
+  (:export :render-inline-markdown))
+
 (defpackage panettone.irc
   (:use :cl :usocket)
   (:export :send-irc-notification))
@@ -42,7 +47,8 @@
 (defpackage panettone
   (:use :cl :klatre :easy-routes :iterate
         :panettone.util
-        :panettone.authentication)
+        :panettone.authentication
+        :panettone.inline-markdown)
   (:import-from :defclass-std :defclass/std)
   (:import-from :alexandria :if-let :when-let :switch :alist-hash-table)
   (:import-from :cl-ppcre :split)