Add a /wip slash command

CHANGELOG

app/controllers/projects/merge_requests_controller.rb

app/models/merge_request.rb

app/services/merge_requests/base_service.rb

app/services/merge_requests/update_service.rb

app/services/slash_commands/interpret_service.rb

doc/user/project/slash_commands.md

spec/controllers/projects/merge_requests_controller_spec.rb

spec/features/issues/user_uses_slash_commands_spec.rb

.../features/merge_requests/user_uses_slash_commands_spec.rb

spec/models/merge_request_spec.rb

spec/services/slash_commands/interpret_service_spec.rb