[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/structs/issue.go

@@ -47,6 +47,7 @@ type Issue struct {
 	OriginalAuthorID int64      `json:"original_author_id"`
 	Title            string     `json:"title"`
 	Body             string     `json:"body"`
+	Ref              string     `json:"ref"`
 	Labels           []*Label   `json:"labels"`
 	Milestone        *Milestone `json:"milestone"`
 	Assignee         *User      `json:"assignee"`