Additional API support for labels (#3290)

* Add API support for labels.

* Error handling for adding/replacing multiple issue labels

* Revisions to function names and error handling. Use issue.ClearLabels in replace/clear functions

* Additional code cleanup

routers/api/v1/convert/convert.go

@@ -129,6 +129,7 @@ func ToDeployKey(apiLink string, key *models.DeployKey) *api.DeployKey {
 
 func ToLabel(label *models.Label) *api.Label {
 	return &api.Label{
+		ID:    label.ID,
 		Name:  label.Name,
 		Color: label.Color,
 	}