add kaminari config with default and max per_page values

config/initializers/kaminari_config.rb

+Kaminari.configure do |config|
+  config.default_per_page = 20
+  config.max_per_page = 100
+  # config.window = 4
+  # config.outer_window = 0
+  # config.left = 0
+  # config.right = 0
+  # config.page_method_name = :page
+  # config.param_name = :page
+end