typeaheads: Change active <li> to have white text again.

This was accidentally changed due to an inheritance problem of
`.typeahead.dropdown-menu a` overriding the default bootstrap
selector `.dropdown-menu .active > a` with its color styling
preferences.

static/styles/zulip.css

@@ -1749,6 +1749,10 @@ blockquote p {
     color: inherit;
 }
 
+.typeahead.dropdown-menu .active {
+    color: #fff;
+}
+
 .nav .dropdown-menu:after {
     position: absolute;
     width: 0px;