Fix #3315: Release dont use tag creation date (#3374)

* Fix #3315: Release dont use tag creation date

* Simplify code and apply gofmt

* remove useless block (ctx.Repo.GitRepo.GetTag) on EditReleasePost

* apply gofmt on modified files

models/release.go

@@ -39,7 +39,9 @@ type Release struct {
 }
 
 func (r *Release) BeforeInsert() {
-	r.CreatedUnix = time.Now().Unix()
+	if r.CreatedUnix == 0 {
+		r.CreatedUnix = time.Now().Unix()
+	}
 }
 
 func (r *Release) AfterSet(colName string, _ xorm.Cell) {