[API] Add Ref to Issue (#13946)

* Add Ref to api.Issue

This PR adds Ref to api.Issue and adds the backend parts to the migrator.

Fix #13918

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/migrations/base/issue.go

@@ -15,6 +15,7 @@ type Issue struct {
 	PosterEmail string
 	Title       string
 	Content     string
+	Ref         string
 	Milestone   string
 	State       string // closed, open
 	IsLocked    bool