Avoid mutating a prop in noteable_note.vue
Follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24801#note_136778519:
noteable_note.vue
mutates the note
prop: https://gitlab.com/gitlab-org/gitlab-ce/blob/e927833b941122f25252712bc68b37041b38ba2c/app/assets/javascripts/notes/components/noteable_note.vue#L182
This is against the way how Vue works and we need to fix it: https://vuejs.org/v2/guide/components-props.html#One-Way-Data-Flow