SNM Version 2.1.1

I'm happy to announce a new stable version of SNM. The contributor list is
growing, now featuring

@eqyiel
@jbboehr
@phdoerfler
@erosennin
@dotlambda
@griff
@geistesk
@matthewbauer
@Infinisil
@danbst

Thanks everyone!

Regressions

• SNM in version 2.1 switches from system users to virtual users. In principle
  upgrading should be smooth, however
  

BACKUP YOUR MAIL DIRECTORY BEFORE UPGRADING

If you rely on email accounts being system users, consider staying on SNM
v2.0.4 or adding the mail accounts as system users.

• If you have more than 100 Gb of mails in your post boxes, you'll need to
  add a quota field to your account (see below: New Features - Quotas)

• If you use Dovecot 2.3 you can to specify the diffie-hellmann bit length
  via

    dhParamBitLength = 2048;
  

  The default is 2048 should be fine. If you want extra security you can
  specify 4096 but be aware that it can take a very long time to create
  the file.

New Features

• Quotas: You can now specify how much disk space each mail account is allowed
  to use.
  loginAccounts = { "user1@example.com" = { hashedPassword = "..."; aliases = [ ... ]; quota = "50M"; }; };
  

The above snippet allows user1@example.com to store up to 50 Mb of data
in her post boxes. Valid units are k/M/G/T with the obvious meanings. If
left blank it will default to a quota of 100 Gb.

• Manage Sieve: You can now optionally allow your users to change, add, remove
  sieve scrips via Dovecot's manage sieve by setting
  enableManageSieve = true;
  

This should work with standard mail clients such as Thunderbird. It will open
port 4190 in the firewall of your sever.

Thanks @erosennin

Bug fixes

• Improved / fixed documentation
  
• Add DNS resolver to stop rspamd from complaining
  
• Avoid installing clamav if virus scanning is turned off
  
• Add support for dovecot 2.3 (nixos-unstable)
  
• Tests have been cleaned up