Limit review replies to plugin authors

[mrfoxtalbot]

We ask users not to pile on existing reviews and to write their own instead. I would propose restricting review replies to the plugin authors only.

By excluding other users from commenting, we could leave review replies always open for plugin authors and this would fix #3505.

This is related to #2884

[tellyworth]

This seems like a good idea. Given that it would limit existing behaviour, I think it needs some discussion and consensus before we implement it.

[mrfoxtalbot]

I have shared a more ambitious approach in #7064.