Issue Summary

If pull request has a reviewer added to them that no longer has access to the repository, it's impossible to perform the following actions on that Pull Request:

• Edit the PR title;
• Edit the PR's body
• Merge

Steps to Reproduce

1. Create a new pull request, and ad some reviewers to it
2. Remove one of the reviewers from the repository
3. Attempting to perform any of the actions highlighted above will throw a something went wrong error message on the UI. Checking the requests made on the devtools console reveal a message like the following:

   {"type":"error","error":{"fields":{"reviewers":["John Doe is not a member of this workspace and cannot be added to this pull request"]},"message":"reviewers: John Doe is not a member of this workspace and cannot be added to this pull request"}}
   

Expected Results

If a user is no longer part of a workspace, they will get automatically removed from all Pull requests they are the reviewer of.

Actual Results

The user will still show up as a reviewer of the PR even after being removed from the repository, causing all PRs they are added as a reviewer to be blocked until that user is removed.

Workaround

The current workaround is to remove the reviewer manually from PRs that are blocked due to this problem.

This will not work if the reviewer previously approved the PR. In this case, the only workaround is to decline the PR and create a new one.