Allow users to set it once for all networks.
Part of this work has been done in a14f646135c0 ("Add per-user realname setting").
I'm not sure it's worth it adding a per-user username setting, given how little IRC servers do with these and given they're overridden by ident.
With no account, we connect you to our IRCd as ~firstname.lastname@example.org.
Registered, you're email@example.com, and can use BOUNCER to connect to other networks.
Motivation is to explore federation / identity based around that.
But, I thought appearing as firstname.lastname@example.org to other networks would be useful for your service, too.
For access lists, bans, knowing who you're talking to, etc.
Added a nickname setting. Decided against adding a username setting.