Issue "remember me" with multiple logins

[the-djmaze]

When using the "remember me" feature it will store the login data on the server in a file. /_data_/_default_/storage/cfg/…/…/sign_me This file is encrypted using the APP_SALT and 'rltoken cookie'.

The rlsmauth cookie should store a custom salt key (not using rltoken) to decode the encrypted data.

It does not solve the issue where multiple people/browsers use the same account. There can only be 1 "remember me", not multiple. Read https://github.com/the-djmaze/snappymail/issues/126#issuecomment-962981582 regarding the multple "remember me" issue.