Allow `r? @ghost`

[Kobzol]

People often do this after a PR is created, but it just results in an error from triagebot. It should ideally unassign the currently assigned reviewer.

[apiraino]

(as discussed with Jakub) In case someone requests r? @ghost after a PR has already been assigned, the triagebot should:

• unassign the reviewer
• suggest the command @rustbot unassign (docs), so the next time the user knows what to do instead.