Render new-dashboard pagination according to current page route

fixes #1613 

The model validation was (silently) preventing the pagination model from being updated as expected, because pagination page and data retreat (e.g. total count, used in the validation) were not set at the same time. Removing this check makes it work as expected.

Also simplified the router model a little, the model's attributes will be set upon the `Backbone.history.start()` call anyway, since the current route's handler sets up things according to current history fragment.