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).
- Enable notification mails for repo activity.
- Put setting for default repo units into "repository" category as the
  "repo" category doesn't exist.
- Also disable all repo units except code, as they mostly aren't needed
  for private repos and can be easily enabled on-demand.

config/hosts/forgejo/forgejo.nix

@@ -39,7 +39,7 @@
         ENABLE_NOTIFY_MAIL = true;
       };
       repository = {
-        DEFAULT_REPO_UNITS = "repo.code,repo.issues,repo.pulls";
+        DEFAULT_REPO_UNITS = "repo.code";
         ENABLE_PUSH_CREATE_USER = true;
         ENABLE_PUSH_CREATE_ORG = true;
       };