10348 add decimal custom field

[arthanson]

Fixes: #10348

Adds custom decimal field type and associated tests.

[jeremystretch]

Great work! The only issue I see at the moment is that decimal values are being stored as strings, rather than decimals/floats. Need to dig into this some more.

[jeremystretch]

The only issue I see at the moment is that decimal values are being stored as strings, rather than decimals/floats.

Ok, tracked this down. The custom_field_data JSONField on each model is using DjangoJSONEncoder, which explicitly encodes decimals as strings.

Ideally, we should save these as raw JSON numbers. I'm not sure of the impact on precision, however it's probably acceptable.

If storing decimal values as JSON numbers is not tenable, an alternative would be to store them as strings but convert to decimals under CustomField.deserialize().

[jeremystretch]

This should be about ready to go. We still need to resolve the validation issue with MultiValueDecimalFilter that @arthanson called out in ad6d4f8, but that should be addressed separately as it may have implications for other filters.