DatabaseExecutionHelper methods should not be static

In particular, `toInternalModel` should be called from an instance within the associated Repository.

This is used in the Database execution implementations:
* [DatabaseReportExecutionRepository](https://github.com/ArpNetworking/metrics-portal/blob/master/app/com/arpnetworking/metrics/portal/reports/impl/DatabaseReportExecutionRepository.java)
* DatabaseAlertExecutionRepository (WIP)