Accept use domain different from web access

[lumenpink]

Summary

In Mastodon we can set LOCAL_DOMAIN for the domain part of ActivityPub users and WEB_DOMAIN for the web access part of it.
This way we can use any domain for access mastodon but the users still match the main domain (like email, for example).
We can have an user@example.com email, but access it from webmail.example.com.

As we can see in https://docs.joinmastodon.org/admin/config/ there are an option for it.

And this doc explain how to do it https://github.com/felx/mastodon-documentation/blob/master/Running-Mastodon/Serving_a_different_domain.md

[mikekasprzak]

Just chiming in, this is a very desirable feature for individuals or teams that wants to treat Misskey or Mastodon like they treat email. You have some main website (myteam.com). You can run your email or Matrix server on subdomains (mail.myteam.com and matrix.myteam.com respectfully). In both cases I can message people @myteam.com, but with Misskey I'm forced to use @subdomain.myteam.com. Criticisms of WEB_DOMAIN and LOCAL_DOMAIN as variables aside, the intent is still good.

[zliebersbach]

Relevant W3C spec here: https://www.w3.org/community/reports/socialcg/CG-FINAL-apwf-20240608/#reverse-discovery

I will have a go at implementation!