[Merge] Enable users to select tax rate when merging expenses, auto-calculate tax amount.

[garrettmknight]

Part of the Merge Expenses project

Main issue: https://github.com/Expensify/Expensify/issues/496985

Feature Description

When merging two expenses that have tax rates selected, we'll do the following:

• To ensure there are no conflicts between the user selecting a tax rate and a tax amount that doesn’t compute, we’ll only allow users to select ‘tax rate’ in the details to keep.
• Where tax rates for the two expenses differ, allow the user to select the tax rate in the details page.
• Show the resulting tax in the confirm page, applying the tax rate to the selected amount.

This method can still result in some conflicts, but they also exist on Classic:

• The selected tax rate could be invalid based on the workspace the selected report is on.
• The selected tax rate could conflict with category rules for tax rates.

Manual Test Steps

1. Create two expenses with different expense amounts and tax rates
2. More > Merge
3. Select one of the tax rates and an amount in the details selector page
4. In the cofirm page, ensure that the rate and amount are used to calculate the resulting tax amount.

Automated Tests

Issue Owner

Current Issue Owner: @dominictb

[melvin-bot]

Triggered auto assignment to @RachCHopkins (Bug), see https://stackoverflow.com/c/expensify/questions/14418 for more details. Please add this bug to a GH project, as outlined in the SO.

[RachCHopkins]

Can someone tell me please, who is the C+ on this one?

[dominictb]

I'll implement this while @hoangzinh is the C+.

[melvin-bot]

@hoangzinh Whoops! This issue is 2 days overdue. Let's get this updated quick!