Remove configuration for username and password length

[link2xt]

Usernames should only be created with nine characters. Existing users should still work, but if they are deleted they cannot be recreated.

This is a replacement for https://github.com/chatmail/relay/issues/697, you can read the reasons for having these restrictions, we want to generate usernames and passwords on the client.

In the core dcaccount: will be allowed to contain just the domain name of chatmail relay instead of HTTPS URL: https://github.com/chatmail/core/pull/7360 Once this is deployed and clients support the new scheme, we can replace the URL on the web page, but this should not be done immediately.