Find a way to label PRs with the relevant code owners.

[seanpmorgan]

Currently we assign maintainers for incoming PRs with CODEOWNERS. However, we're limited to tagging users with write access.

Seeing as a majority of our maintainers are submodule and not repository maintainers, it would be nice to devise a strategy for tagging submodule maintainers (even if it's just an @). Possibly would need to be a bot.

[gabrieldemarmiesse]

We had a similaire issue in keras-contrib. Maybe we can port some of the code from there: keras-team/keras-contrib#416