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/panettone.lisp

@@ -192,7 +192,7 @@
            (:a :href (format nil "/issues/~A" issue-id)
                (:p
                 (:span :class "issue-subject"
-                       (who:esc (subject issue))))
+                       (render-inline-markdown (subject issue))))
                (:span :class "issue-number"
                       (who:esc (format nil "#~A" issue-id)))
                " - "
@@ -329,7 +329,7 @@
         (issue-status (status issue)))
     (render ()
       (:header
-       (:h1 (who:esc (subject issue)))
+       (:h1 (render-inline-markdown (subject issue)))
        (:div :class "issue-number"
              (who:esc (format nil "#~A" issue-id))))
       (:main