Only show re-request review button to users who have permission

What does this MR do and why?

Currently the re-request review button in the MR sidebar shows for any user (even a signed out user) because the source code is checking whether the assigned reviewer has permission, not whether the current user has permission. This MR fixes that.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• #499697 (closed)
• #is-this-known discussion

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Check out this branch
2. In the GDK, visit an MR that has any assigned reviewer that has completed some review activity
3. You should see, when signed in as a user who can edit the MR, that the "re-request review" button shows next to the reviewer in the sidebar
4. Copy the URL of the MR and open it in an incognito window
5. You should see the MR and you should not be signed in. You should see that the "re-request reivew" button is not present.