Allow multiple servers to share a webmail instance

[thomasrudin]

Suffix server name in mail addresses

admin@pandorabox
someuser@whatever

Prerequisite: #4

TODO

• Backwards compat
• Mails without suffix?
• Bundle multiple addresses
• Spam?
• With Add attachment feature #2: item restrictions/limits

[rubenwardy]

This would be pretty awesome. I don't particular care for inter-server communication, however, but sharing an instance can be convenient

There are two different auth methods I see:

• By default, the player would sign into each server and have an account switcher tab
• Shared authentication when the servers used a shared PostgreSQL auth database (Coming in 5.1.0). The player should only need to login once in this case