Refactor slash command definition

app/controllers/projects_controller.rb

app/services/issuable_base_service.rb

app/services/notes/create_service.rb

app/services/notes/slash_commands_service.rb

app/services/projects/autocomplete_service.rb

app/services/projects/participants_service.rb

app/services/slash_commands/interpret_service.rb

lib/gitlab/slash_commands/command_definition.rb

lib/gitlab/slash_commands/dsl.rb

lib/gitlab/slash_commands/extractor.rb

spec/lib/gitlab/slash_commands/extractor_spec.rb