Email aliases are not valid login identifiers

[tituspijean]

Describe the bug

Discovered while experimenting with aliases as suggested in this thread: https://forum.yunohost.org/t/same-user-name-different-domain/19837

1. Setup alice account, with email alice@domain.example
2. Setup bob account with email bob@domain.example and alias alice@otherdomain.example

SnappyMail requires one to log in with their full email address, not only the username. So alice would be logging in with alice@domain.example and bob with bob@domain.example...

What about alice@otherdomain.example? It is not really considered as an email address by the login form of SnappyMail, rather as alice on otherdomain.example server, so it logs the user in as alice and not bob. (Discovered because I used the same password for both test accounts).

Expected behavior

I guess we can't do much here, maybe add a notice in the README like "aliases are not valid identifiers to log in". And maybe a suggestion for upstream: "do not put 'email' as placeholder in the login form".