Be slightly stricter & smarter about the Zulip command "modifier" `as <username>`

[fmease]

This is definitely P-low :P Just some QA fun :D

1. Re. smarter: If user $user issues as $user $command (i.e., where the target user is the user itself), triagebot should just treat it as $command and not send another message informing the issuing user that they executed a command on their own behalf
2. Re stricter: Disallow as <user0> as <user1> <command> (etc. at infinitum). Currently, it 'pings' all users in the 'chain'.

Feel free to close

[Kobzol]

Related: #2004

I was thinking of rewriting the whole Zulip command handling with Clap, and implement "read-only" commands that wouldn't ping if you use as.

[Kobzol]

Fun fact: nested as <username> commands recursively called a function, so if you had enough as, you could probably overflow triagebot's stack xD No more!