Markdown: add <details>+<summary> to whitelist

This allows creating collapsible sections, which will probably be
especially useful in wiki pages.

tildes/scss/_base.scss

@@ -50,6 +50,11 @@ dl dd {
   font-size: 0.7rem;
 }
 
+details:not([open]) {
+  cursor: pointer;
+  display: inline-block;
+}
+
 // Fixes a Chrome bug that seems to treat children of <details> as having
 // box-sizing: content-box. See Known issues: https://caniuse.com/#feat=details
 details * {

tildes/scss/_placeholders.scss

@@ -11,6 +11,12 @@
     }
   }
 
+  details {
+    margin-bottom: 0.4rem;
+    padding: 0.2rem 0.4rem;
+    border: 1px dotted;
+  }
+
   hr {
     margin-left: 0;
     max-width: $paragraph-max-width;

tildes/scss/modules/_topic.scss

@@ -182,7 +182,7 @@
     line-height: 0.8rem;
   }
 
-  summary::after {
+  > summary::after {
     font-style: italic;
     content: "Re-collapse topic text";
     display: none;

tildes/tildes/lib/markdown.py

@@ -63,6 +63,7 @@ HTML_TAG_WHITELIST = (
     "br",
     "code",
     "del",
+    "details",
     "em",
     "h1",
     "h2",
@@ -80,6 +81,7 @@ HTML_TAG_WHITELIST = (
     "small",
     "strong",
     "sub",
+    "summary",
     "sup",
     "span",
     "table",
@@ -94,6 +96,7 @@ PROTOCOL_WHITELIST = ("http", "https", "mailto")
 
 HTML_ATTRIBUTE_WHITELIST_DEFAULT = {
     "a": ["href", "title"],
+    "details": ["open"],
     "ol": ["start"],
     "td": ["align"],
     "th": ["align"],