Use toast instead of flash for quick actions in comments
<!-- This template is a great use for issues that are feature::additions or technical tasks for larger issues.--> ### Proposal Currently, when we submit a quick action command (e.g. `/draft`), the frontend shows a message in a form of a flash: ![Kooha-10-13-2021-11-08-07](/uploads/414caeaed0170739bd9343be8a5e59ee/Kooha-10-13-2021-11-08-07.webm) We could change it to toast to match the other notifications: ![Kooha-10-13-2021-11-08-39](/uploads/9b2822a5bc7864dfdb0116f88db277da/Kooha-10-13-2021-11-08-39.webm) This issue came from a discussion on https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71887#note_699338640 /cc @annabeldunstone
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