Fix #3214: Add autocomplete for saved searches.

* Autocomplete the `search:<label>` metatag.

* Make label autocompletion do a prefix match instead of a substring
  match. Example: `search:ar` matches `artists`, not `characters`. This
  is how tags and most other things are autocompleted.

app/assets/javascripts/saved_searches.js

@@ -1,5 +1,16 @@
-$(document).ready(function() {
+Danbooru.SavedSearch = {};
+
+Danbooru.SavedSearch.initialize_all = function() {
   if ($("#c-saved-searches".length)) {
     Danbooru.sorttable($("#c-saved-searches table"));
   }
-});
+}
+
+Danbooru.SavedSearch.labels = function(term) {
+  return $.getJSON("/saved_searches/labels", {
+    "search[label]": term + "*",
+    "limit": 10
+  });
+}
+
+$(Danbooru.SavedSearch.initialize_all);