Improve Crate Owners UX

[golddranks]

Hi, I just used the Manage Crate Owners / Add Owner functionality in crates.io for the first time. I found the UX of the form not to be quite ideal:

• It doesn't detect if the user I'm trying to pass ownership exists at all:

• Because of this, I can't be entirely sure whether I typed the username correctly.
  • I have a nagging feeling, after typoing once, that I might be passing the ownership to a malicious fake user.
  • After page reload, it doesn't show that an invitation has been sent, and there is no way to revoke an invitation in the case I typoed.

While not maybe a priority number one of crates.io, I'd appreciate if these concerns were heard.

[Zexbe]

Yeah, I just sent an invite to the wrong person on accident, and I don't have a way to cancel it.