feat(activitypub): use structure @PreferredUsername@host.tld:port for actors (#9254)

This modifies usernames of ActivityPub accounts to use the @example@example.tld
format with an additional optional port component (e.g. @user@example.tld:42).
This allows accounts from ActivityPub servers with more relaxed username
requirements than those of Forgejo's to interact with Forgejo. Forgejo would
also follow a "de facto" standard of ActivityPub implementations.

By separating different information using @'s, we also gain future
opportunities to store more information about ActivityPub accounts internally,
so that we won't have to rely on e.g. the amount of dashes in a username as
my migration currently does.

Continuation of Aravinth's work: https://codeberg.org/forgejo/forgejo/pulls/4778

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9254
Reviewed-by: jerger <jerger@noreply.codeberg.org>
Reviewed-by: Ellen Εμιλία Άννα Zscheile <fogti@noreply.codeberg.org>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Panagiotis "Ivory" Vasilopoulos <git@n0toose.net>
Co-committed-by: Panagiotis "Ivory" Vasilopoulos <git@n0toose.net>

models/fixtures/user.yml

@@ -1566,9 +1566,9 @@
 
 -
   id: 42
-  lower_name: federated-example.net
-  name: federated-example.net
-  full_name: federated
+  lower_name: "@federated@example.net"
+  name: "@federated@example.net"
+  full_name: "@federated@example.net"
   email: f73240e82-c061-41ef-b7d6-4376cb6f2e1c@example.com
   keep_email_private: false
   email_notifications_preference: enabled
@@ -1576,7 +1576,7 @@
   passwd_hash_algo: ""
   must_change_password: false
   login_source: 0
-  login_name: federated-example.net
+  login_name: "@federated@example.net"
   type: 6
   salt: ""
   max_repo_creation: -1