Allow slack service to send messages on different channels

CHANGELOG

app/controllers/admin/services_controller.rb

app/controllers/projects/services_controller.rb

app/helpers/services_helper.rb

app/models/project_services/slack_service.rb

app/views/admin/services/_form.html.haml

app/views/projects/services/_form.html.haml

...views/projects/services/slack/_service_settings.html.haml

features/steps/admin/settings.rb

spec/models/project_services/slack_service_spec.rb