Assign author and reviewer to forward port if possible

[xmo-odoo]

Don't remember if I looked at this previously or tried it, but I should check if I did and if not probably see into doing that: it can be a bit difficult to find forward port PRs after you've marked notifications as read / deleted emails. Since we don't use assignments a lot at odoo, this would be a fine way to track the things.

[xmo-odoo]

Found where I'd previously looked at this: #110 (comment)

This was mostly targeted at the runbot at the time, and does have some of the possibly problematic considerations: you can only assign someone on an issue / PR if they are "contributors" or have "interacted with".

However thinking about it further it shouldn't be a concern for this: AFAIK any user which has authored a commit in the repo is a contributor. This means the author of the original PR is a contributor for (since we're forward porting their PR), and the approver is likely a contributor as well, given we usually give approval (review) rights to somewhat seasoned employees.

edit: https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/assigning-issues-and-pull-requests-to-other-github-users

You can assign [...] yourself, anyone who has commented on the issue or pull request, anyone with write permissions to the repository, and organization members with read permissions to the repository.