Make sidebar Vuex store available

Add mr_notes/stores

Add issuable warning header to mr

Also separate issuable warning header initializer.

Use graphql to update lock status for mr

Separate some changes into a separate MR.

Use graphql for locking/unlocking issue

Set discussion_locked to false if it's null initially.

Update the edit form related specs

Remove mediator and comments

Also remove wait_for_promises helper

Add errors field for consistency