Added ability to change models number limit manually.

[SuperMasterBlasterLaser]

According to #1369 RailAdmin set number of models that will be uploaded to multiselect forms. In other cases it will load nothing, forcing user to write something on input field to make search.

This feature makes users to be able to set number of limit manually:

 RailsAdmin.config do |config|
      config.association_select_models_number_limit = 500
 end

[mshibuya]

Closing in favor of #2058, which comes with test code.