Add support to migrate from gogs (#14342)

Add support to migrate gogs:

  *  issues
  *  comments
  *  labels
  *  milestones
  *  wiki


Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Andrew Thornton <art27@cantab.net>

modules/migrations/base/milestone.go

@@ -15,5 +15,5 @@ type Milestone struct {
 	Created     time.Time
 	Updated     *time.Time
 	Closed      *time.Time
-	State       string
+	State       string // open, closed
 }