Refactor price quantity data architecture

Problem/Motivation

I am in the process of migrating farmOS v1 instances to v2, and some of these are using the Sale/Purchase log types that were provided in v1 (which are now provided by this module). In reviewing the way these Sale/Purchase logs were used in v1, I'm noticing that the v2 module's migration logic is not exactly correct.

When we created the new "Price quantity", we designed it so that the "value" of the quantity would represent the "total price" of the transaction, and two additional fraction fields were added to price quantities alongside that: "Unit price" (the price per unit) and "Unit quantity" (the number of units). Thus, unit price multiplied by unit quantity should always equal total price (and we even hide the value field on this quantity type and auto-populate it based on that math). In this design, the "Units" of the price quantity is expected to be the currency of the transaction (eg: "USD").

However, looking back at the fields on v1 Sale/Purchase logs, our migration logic is flawed.

These are the v1 fields and their descriptions:

- Quantity - How many were sold?
- Units - What is the unit of measurement for the quantity entered?
- Unit price - (no description)
- Total price - (no description)

The current migration logic provided by this v2 module, however, creates a Price quantity mapped from v1 data as follows:

- Value - "Total price" from v1
- Units - "Units" from v1
- Unit quantity - "Quantity" from v1
- Unit price - "Unit price" from v1

As you can see, most make sense... but "Units" does not. At least not if we are working under the assumption that "Value" will be the total price and "Units" will be the currency.

To illustrate, imagine this Sale log in v1:

- Quantity: 5
- Units: plants
- Unit price: 10
- Total price: 50

The v2 migration logic will create a Price quantity that looks like this:

- Value (aka total price): 50
- Units: plants
- Unit price: 10
- Unit quantity: 5

Proposed resolution

- Remove unit_quantity field.
- Add total_value field.
- Migrate existing quantity value data to new total_value field.
- Migrate existing unit_quantity data to quantity value field.
- Update all Views, migration, theming, logic, etc.

Remaining tasks

(none)

User interface changes

- "Unit quantity" field is replaced with "Total price" field.
- Measure, Value, and Inventory fields are no longer hidden.
- Revert to default quantity theming.

API changes

- Removal of unit_quantity field on price quantities.
- Addition of total_price field on price quantities.

Data model changes

- Removal of unit_quantity field on price quantities.
- Addition of total_price field on price quantities.
- The value field of price quantities no longer represents the "total price" of the quantity. Instead, it represents the "number of units", which is more inline with how other quantity types are used.