User deletion API: confirm by giving user's email rather than by their name?

[fflorent]

See this conversation: https://github.com/gristlabs/grist-help/pull/367/files#r1696590658

It could be more convenient to rather allow deleting a user by giving their normalized email address rather than by giving their name, assuming that there is only a user per login (so by email address, because they are unique in the logins table).

What do you think?