Allow customizing `displayName` and `displayShortName` in autoconfig

[DrMaxNix]

Summary

The autoconfig http endpoint (and maybe also others) are currently returning hard coded values for the server's name:

<displayName>A mailcow mail server</displayName>
<displayShortName>mail server</displayShortName>

I suggest replacing these strings with something the user can configure:

1. Simple Solution: Load displayName and displayShortName from php variables, which can be configured using the already existing vars.local.inc.php file.
2. Coherent and more Robust Solution: Add new inputs to the System > Configuration > Options > Customize Tab called Server Name/Server Short Name or something similar, which will be used by the autoconfig endpoint and can also be used in other places where the server should have a name. Maybe these could be merged with "mailcow UI" website title/"mailcow UI" name, but I don't know how much breaking this change would be.

Motivation

When you have multiple mail servers configured in your mail client (maybe even multiple Mailcow servers), it may be very difficult for average users to understand, what "A mailcow mail server" means. Having your organization's name show up instead will be much more comprehensive!