Merge request review stages

When an MR goes into review, suggestions or discussions are made, code is changed, more suggestions and discussion are made.

It would be nice to group the activity that occurs during each of these review stages. It will allow us to reduce large discussion tabs into something more useful; we can collapse old review stages and expand the latest.

I don't think its an easy task to design it on top of our single timeline but would love to see discussion.

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Example "rally" using slash commands (🏓 🎉)

• /review starts a review stage and assigns to the user if provided.

• /reviewed ends a review stage and assigns back to the author.

• subsequent /review commands start a new review stage and assigns to the previously selected reviewer.

Stage 1

@contributor: @reviewer could you review this?

/review @reviewer

system notes, comments, discussion, suggestions...

@reviewer: Looking good @contributor, I left a few suggestions.

/reviewed

Stage 2

system notes, comments, discussion, suggestions...

@contributor: @reviewer Thanks, that was a good find!

/review

system notes, comments, discussion, suggestions...

@reviewer: Looking good @contributor, on to @maintainer.

/assign @maintainer

Example of ping pong.

Screen_Recording_2019-03-01_at_20.08.27