triagebot: enable issue transfer

[lolbinarycat]

I believe it was already possible to transfer issues into the reference repo without this, but this will allow transferring issues both ways.

Issue transfer is still only available to members of teams and certain working groups.

https://forge.rust-lang.org/triagebot/transfer.html

[traviscross]

Thanks for the PR. @ehuss and I are talking about this on the lang-docs call, and while we're not opposed to adding this, we're struggling to think of a case where transferring an issue out of the Reference would be the right thing to do.

Even if, e.g., someone filed something against the Reference and we decided that it was actually a bug in the compiler, it would seem better to just close and refile. The issue filed against the Reference is likely to be written in such a way that it wouldn't make a good issue against r-l/r on its own.

We're curious to know if there are other cases one might have in mind here.