Batch comments on merge requests

What this MR do

• Create a draft_notes table which is similar to notes table
• When user hits to publish draft we transfer all draft_notes to notes table

Important things to cover

• Draft notes can be created in a diff file or not
• Draft notes should have a position that works just like it does in DiffNotes
• When draft note is on an present discussion we should consider the discussion_id when publishing
• When draft note is on a new discussion we should generate a new discussion ID

obs: It is not needed to group draft notes by discussion_id for now, as described on the #1984 (closed) it is only allowed to have one draft note per discussion.

Important things to test

• When draft is published check if notes are working on image diffs and text diffs.

Important implementation details

• When sending in a DraftNote as a reply to a discussion, in_reply_to_discussion_id should be sent, not discussion_id
• Please send resolve_discussion according to this table:

Current Discussion state	Desired Discussion state	resolve_discussion value
unresolved	resolved	true
unresolved	unresolved	false
resolved	unresolved	false
resolved	resolved	true

ToDo

• [Backend] Adding license check for GitLab Premium
• [Backend] Adding publishing service
• [Backend] Keep position + discussion id for all draft notes
• [Backend] Render the DraftNote HTML via Banzai
• Backports for CE

For Frontend, please check separate MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7376

Edge cases / Follow up?

• What happens when a draft is saved in a discussion that gets deleted?