Clarify "identifier" syntax in UI fields

Our UI fields all have an "identifier" (e.g. `plot.geom`) which we use for communicating values between client and server. Be more consistent about how the first part is handled:
* Spell it consistently, as the model name with a lowercase first letter (e.g. `benefitCurrencyConversion`)
* Call it the "object name" not the "model name", since it is capitalized differently than the model name
* Add and use utilities `to_model_name()` and `to_object_name()`