Configurable username format for webhook relays

[austinhuang0131]

Originally posted at https://geekdom.social/@bigolifacks/112859769835968061 (not me); the concerning code is

https://github.com/mautrix/discord/blob/035f2a408b4a6cf73acbf168dff5d16198930ee2/portal.go#L1436-L1441

so eg. allows the bridge to always include MXID in the webhook display name.

[tulir]

Making it configurable doesn't really solve anything. I'll probably include proper disambiguation in megabridge, but I couldn't find any Go libraries for UTS #39 confusable detection, so will need to write one first

[Raymo111]

@tulir Making it configurable solves the problem of people not knowing where the webhook is bridging from (enables appending something like | from CC Chat. I think having it would be a good feature even without the megabridge disambig.