tweak forgejo service configuration a bit making it nicer

- Enable Git LFS support, since it's nice to have.
- Enable offline mode to avoid relying on CDNs (and to not use
  Gravatar).
- Disable internal sign-in, since OIDC is used and forced for
  registrations anyway.
- Enable notification mails for repo activity.
- Put setting for default repo units into "repository" category as the
  "repo" category doesn't exist.

config/hosts/forgejo/forgejo.nix

@@ -33,6 +33,7 @@
       };
       service = {
         ALLOW_ONLY_EXTERNAL_REGISTRATION = true;
+        ENABLE_INTERNAL_SIGNIN = false;
         DEFAULT_USER_VISIBILITY = "limited";
         DEFAULT_KEEP_EMAIL_PRIVATE = true;
         ENABLE_BASIC_AUTHENTICATION = false;