[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>

templates/swagger/v1_json.tmpl

@@ -13831,6 +13831,10 @@
         "pull_request": {
           "$ref": "#/definitions/PullRequestMeta"
         },
+        "ref": {
+          "type": "string",
+          "x-go-name": "Ref"
+        },
         "repository": {
           "$ref": "#/definitions/RepositoryMeta"
         },