Fix situations in which instance actor can be set to a Mastodon-incompatible name

[ClearlyClaire]

Fixes #20820

• instead of using instance-name@instance-name, use internal.actor@instance-name for the name of new instance actors, this still makes the instance actor use a name that can't be used by local user
• validate actor name to catch such issues
• automatically reset the instance actor name if it is found to include a :, as it would be incompatible with most of the existing fediverse

[Gargron]

Is : the only situation where the username would be incompatible? Since LOCAL_DOMAIN is supposed to be punycode, there should not be others. Perhaps we should add a check to LOCAL_DOMAIN to not allow port inclusion in production, or just exclude the port from the actor name? I'm not sure how I feel about the internal.actor username 😅

[ClearlyClaire]

Is : the only situation where the username would be incompatible? Since LOCAL_DOMAIN is supposed to be punycode, there should not be others.

There might be other, it might indeed be better to check for the regexp, but i wanted something simple that matched the cases I'm aware of.

Perhaps we should add a check to LOCAL_DOMAIN to not allow port inclusion in production, or just exclude the port from the actor name?

localhost would be an issue since that can clash with a normal user, so i figure it's better if we use something fixed with a . in it

I'm not sure how I feel about the internal.actor username sweat_smile

mastodon.internal? mastodon.server? internal.mastodon? internal.server?