fix: only send Forgejo Actions notifications to one user (#8227)

- If the run was attributed to a system user (which is the case for scheduled runs for instance), ignore it and fallback to the mail of the owner. System users may have email addresses, but they are not to be used.
- If the owner is a system user or an organization with no email associated with it, do nothing.
- If a user with an email exists, check if they did not disable notifications and send the email.

Refs: https://codeberg.org/forgejo/forgejo/issues/8187
Refs: https://codeberg.org/forgejo/forgejo/issues/8233

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.

### Documentation

- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8227
Reviewed-by: Christopher Besch <mail@chris-besch.com>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

models/user/user_system.go

@@ -12,6 +12,13 @@ import (
 	"forgejo.org/modules/structs"
 )
 
+// IsSystem returns true if the user has a fixed
+// negative ID, is never stored in the database and
+// is generated on the fly when needed.
+func (u *User) IsSystem() bool {
+	return u.IsGhost() || u.IsActions()
+}
+
 const (
 	GhostUserID        = -1
 	GhostUserName      = "Ghost"