Refactor design_discussion.vue to use ApolloMutation component

Summary

Refactor ee/app/assets/javascripts/design_management/components/design_notes/design_discussion.vue to use <ApolloMutation> component.

Improvements

design_discussion.vue uses Apollo mutation with calling this.$apollo.mutate. Refactoring it to use ApolloMutation component will help us split the logic more clearly between template and script parts and will also make our codebase more consistent

Risks

There are no significant risks on this refactoring

Involved components

• ee/app/assets/javascripts/design_management/components/design_notes/design_discussion.vue

Optional: Missing test coverage

• ee/spec/frontend/design_management/components/design_notes/design_discussion_spec.js