Add support for nested groups to admin routing

app/views/admin/dashboard/_head.html.haml

app/views/admin/dashboard/index.html.haml

app/views/admin/groups/_group.html.haml

app/views/admin/groups/show.html.haml

app/views/admin/projects/index.html.haml

app/views/repository_check_mailer/notify.html.haml

app/views/repository_check_mailer/notify.text.haml

config/routes/admin.rb

features/steps/shared/paths.rb

spec/features/admin/admin_groups_spec.rb

spec/features/admin/admin_projects_spec.rb

spec/features/security/admin_access_spec.rb

spec/routing/admin_routing_spec.rb