fix: aggregate deleted team as ghost team (#7987)

- If a review was requested from a deleted team, use the ghost team for the comment aggregator.
- Resolves Codeberg/Community#1952
- Unit test added.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7987
Reviewed-by: Beowulf <beowulf@beocode.eu>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

models/organization/team.go

@@ -292,3 +292,11 @@ func FixInconsistentOwnerTeams(ctx context.Context) (int64, error) {
 
 	return int64(len(teamIDs)), nil
 }
+
+func NewGhostTeam() *Team {
+	return &Team{
+		ID:        -1,
+		Name:      "Ghost team",
+		LowerName: "ghost team",
+	}
+}