Improve Zulip user impersonation #2004
Description
Triagebot currently allows users to send Zulip commands on behalf of other users using as <username> <command>. Triagebot will currently send a message to <username>, letting them know that someone else has executed a command on their behalf. However, this can be quite spammy if the command is "safe", e.g. if you want to read the review preferences and review queue capacity of some other contributor.
We should refactor the commands to be more "first-class" (e.g. represent commands with some struct or trait) and distinguish "safe/read-only" commands (for which triagebot would not notify the user) and "unsafe/modifying" commands (for which triagebot would notify the impersonated user).