* Add database model for defining custom units
- Database model
- DRF serializer
- API endpoints
* Add validation hook
* Custom check for the 'definition' field
* Add settings page for custom units
- Table of units
- Create / edit / delete buttons
* Allow "unit" field to be empty
- Not actually required for custom unit definition
* Load custom unit definitions into global registry
* Docs: add core concepts page(s)
* Add some back links
* Update docs
* Add unit test for custom unit conversion
* More unit testing
* remove print statements
* Add missing table rule
