Work on failed outbound requests and move type migrators

- DB type migrators are now in separate file, in preparation for full
  custom sql migration statements
- Start work on handling failed outbound requests stored in the db

cmd/model-gen/main.go

@@ -54,6 +54,7 @@ func main() {
 	g.ApplyInterface(func(models.IAccessToken) {}, models.AccessToken{})
 	g.ApplyInterface(func(models.INote) {}, models.Note{})
 	g.ApplyInterface(func(models.IUserToUserRelation) {}, models.UserToUserRelation{})
+	g.ApplyInterface(func(models.IFailedOutboundRequest) {}, models.FailedOutboundRequest{})
 
 	log.Info().Msg("Extra features applied, starting generation")
 	g.Execute()