Implement support for the yearly 1604-C. It has two parts:
- One summary sheet which recaps the monthly 1601-C declarations done
during the year.
- One DAT file containing the detailled income information of all
employees in the company. This is similar to a list of all the forms
2316 for the company.

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As we are adding support for the 1604-C, we need even
more details from the previous employment of the
employees which joined during the year.

As it starts to make for a lot of fields, we'll move
them to a new model to avoid adding too many columns
on the version/employee model.

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Currently, the computation of _compute_work_contact_details
is not correct in the Philippines.
The compute should set the VAT, but it is missing from the
depends causing it not to work as expected.

Also add a computed field to store the branch code of the
employee, allowing to easily fetch it when needed in the
declarations.

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Improve the performance of the declarations test by
batching as much as possible during the setup of the
tests.
Also merge the foreign employee and high earner 2316
tests in once to reduce duplications.

This reduces the time it takes to run the current test
suite by around 25% (calculated on the fly based on logs,
to take with a grain of salt) and simplify the individual
test implementations.

It will also help keep the performances in line when adding
tests for 1604-C which require to create 1601-C and 2316 for
the period.

task-6278100