autocomplete: make number of results configurable (partly).

Add `Danbooru.Autocomplete.MAX_RESULTS` param that controls the number
of results returned by autocomplete.

This does not work with tag autocomplete for Builders. Builder
autocomplete still needs to be refactored and unified with Member
autocomplete.

Also fix a bug in the /saved_searches/labels endpoint where the limit
param wasn't respected.

app/controllers/saved_searches_controller.rb

@@ -7,7 +7,7 @@ class SavedSearchesController < ApplicationController
   end
 
   def labels
-    @labels = SavedSearch.search_labels(CurrentUser.id, params[:search])
+    @labels = SavedSearch.search_labels(CurrentUser.id, params[:search]).take(params[:limit].to_i || 10)
     respond_with(@labels)
   end