MM-25394 session expired push notifications (#14732)

* new job type created that checks for expired mobile sessions and pushes notifications.

* only send session expired notifications if ExtendSessionLengthWithActivity is enabled.

* includes schema change:  field added to Sessions table

model/job.go

@@ -19,6 +19,7 @@ const (
 	JOB_TYPE_LDAP_SYNC                      = "ldap_sync"
 	JOB_TYPE_MIGRATIONS                     = "migrations"
 	JOB_TYPE_PLUGINS                        = "plugins"
+	JOB_TYPE_EXPIRY_NOTIFY                  = "expiry_notify"
 
 	JOB_STATUS_PENDING          = "pending"
 	JOB_STATUS_IN_PROGRESS      = "in_progress"
@@ -59,6 +60,7 @@ func (j *Job) IsValid() *AppError {
 	case JOB_TYPE_MESSAGE_EXPORT:
 	case JOB_TYPE_MIGRATIONS:
 	case JOB_TYPE_PLUGINS:
+	case JOB_TYPE_EXPIRY_NOTIFY:
 	default:
 		return NewAppError("Job.IsValid", "model.job.is_valid.type.app_error", nil, "id="+j.Id, http.StatusBadRequest)
 	}