Add Mod Log model

https://eaflood.atlassian.net/browse/WATER-4563
https://eaflood.atlassian.net/browse/WATER-4564
https://eaflood.atlassian.net/browse/WATER-4565

> Part of the work to display a licence's history to users (mod log)

Now we have a table to hold the imported NALD mod log records we need a model to access it, and relationships to access the mod logs from charge, licence, and return versions.

Once added and linked we can then complete our work to expose this information, for example.

- [Build new licence history page](#1182)
- [Update view return version to include mod log info](#1261)

Author: Cruikshanks