Move styles for collapsed <details> block

Having it in _base.scss was breaking the mobile layout because it ended
up affecting the text-topic excerpts. This should be safer, since it was
really only intended to affect the new <details> blocks possible inside
user text anyway.

tildes/scss/_base.scss

@@ -50,11 +50,6 @@ 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

@@ -15,6 +15,11 @@
     margin-bottom: 0.4rem;
     padding: 0.2rem 0.4rem;
     border: 1px dotted;
+
+    &:not([open]) {
+      cursor: pointer;
+      display: inline-block;
+    }
   }
 
   hr {