Support <details> collapsing in markdown

The markdown sanitizer's allowlist did not include the <details> and
<summary> tags, so GitHub-style collapsible sections were stripped and
their contents rendered flat. Add both tags to ALLOWED_TAGS and the
"open" attribute to ALLOWED_ATTR, so collapsible sections render and
<details open> starts expanded.

Closes 390f3f2958

src/lib/markdown.ts

@@ -30,6 +30,7 @@ export const sanitizeConfig: Config = {
     "href",
     "id",
     "name",
+    "open",
     "rel",
     "src",
     "target",
@@ -46,6 +47,7 @@ export const sanitizeConfig: Config = {
     "code",
     "dd",
     "del",
+    "details",
     "div",
     "dl",
     "dt",
@@ -67,6 +69,7 @@ export const sanitizeConfig: Config = {
     "radicle-external-link",
     "s",
     "strong",
+    "summary",
     "table",
     "tbody",
     "td",