Implement mention suppression

[Kobzol]

Bors should remove mentions (@<username>) from PR descriptions, which are turned into merge commit bodies, to avoid spamming people on GitHub.

See rust-lang/homu#100 and rust-lang/homu#230 for more details.

[xosnrdev]

I can look into this.

[thaliaarchi]

If this feature were implemented, I'd be interested in testing the grammar of the username matching and porting rust-lang/homu#230 to it.