Add rustbot prioritize command

[spastorino]

Would be nice if we have @rustbot prioritize command that assigns I-prioritize label and also creates a new topic on Zulip's WG-prioritization stream requesting the working group to prioritize the in question issue.

Read more about the discussion about this feature request in this Zulip topic.

[spastorino]

cc @rust-lang/wg-prioritization

[Mark-Simulacrum]

Please also document on this issue the format for the topic (e.g., should it link to the issue?)

A sample topic in Zulip would be good, too.