Use same constant for amount of items per page

app/controllers/admin/groups_controller.rb

app/controllers/admin/projects_controller.rb

app/controllers/application_controller.rb

app/controllers/dashboard/groups_controller.rb

app/controllers/dashboard/milestones_controller.rb

app/controllers/dashboard_controller.rb

app/controllers/explore/groups_controller.rb

app/controllers/explore/projects_controller.rb

app/controllers/groups/milestones_controller.rb

app/controllers/groups_controller.rb

app/controllers/profiles_controller.rb

app/controllers/projects/branches_controller.rb

app/controllers/projects/issues_controller.rb

app/controllers/projects/labels_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/controllers/projects/milestones_controller.rb

app/controllers/projects/tags_controller.rb

app/controllers/projects/wikis_controller.rb

app/controllers/snippets_controller.rb