Use Kaminari configuration if present

First of all, this is not a kaminari vs will_paginate patch but it's [related to](https://github.com/amatsuda/kaminari/issues/162#ref-commit-3cf929f). In that commit amatsuda introduces some configuration variables for kaminari. I'm using them like

    Kaminari.configure do |config|
      config.page_method_name = :pagina
      config.param_name = :pagina
    end

not because of conflict, but for i18n issues. And forem is still calling `page` and using `params[:page]`.
This patch uses that configuration if present, and defaults to `page` if not.

I can add some tests if this approach is welcome.