Add option to lowercase all local_parts / domains

[martbock]

I want to be able to restrict all email addresses (hence all local_parts and domains) to be lowercase-only. This should prevent some user confusion whether the address is case-sensitive or not. Personally, I can't understand people that use uppercase symbols in their email addresses - but hey. Anyways, the option should be configurable with a .env variable. All user input from create/edit forms should automatically be lowercased if the option is enabled.