Add a shared searchable multiselect combobox for team/member pickers

Extracts the "Grant role" member picker's typeahead combobox into a
reusable static/js/searchable-select.js (opt in via data-searchable on
the widget), and reuses it for the news form's teams field -- a proper
multiselect with removable pills and filter-as-you-type, replacing the
plain checkbox list. Needed forwarding widget attrs through the shared
select template, which never passed them to the rendered <select>.

static/css/app.css

@@ -3900,6 +3900,18 @@
       }
     }
   }
+  .empty\:hidden {
+    &:empty {
+      display: none;
+    }
+  }
+  .hover\:opacity-100 {
+    &:hover {
+      @media (hover: hover) {
+        opacity: 100%;
+      }
+    }
+  }
   .sm\:hidden {
     @media (width >= 40rem) {
       display: none;