Add command for relabeling issues from Zulip

[spastorino]

This issue tracks the corresponding issue on triagebot#591.

r? @Elinvynia

[spastorino]

This rust-lang/triagebot#12 may be useful.

[spastorino]

This seems to be now assigned to @LeSeulArtichaut

[spastorino]

Work by @LeSeulArtichaut that may help in this effort lives https://github.com/LeSeulArtichaut/triagebot/tree/zulip-commands