Collapse the right sidebar near PR description with Reviewers, Assignees, Labels, etc. similarly to files tree on the diff view

[pilgunboris]

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While writing PR description and QA steps for the larger PRs it is helpful to open 2 windows side by side with the diff on one side and PR description on the other, so you can go through all the changed files and describe every change.
During this process the sidebar with Reviewers, Assignees, Labels, Projects, etc. is not needed, but it takes up a lot of space (I'm working on 16" MacBook, I guess on smaller screens it is even more painful).

I think it would be really helpful to collapse this section when needed, just like it works today with a changed files tree on the diff view.
This will provide a better experience while working with PR description because you will be able to write whole sentences in a wider window rather than a few words per line in a vertical window.