Enable more flexible field mapping

Given that

• some fields are not always returned from the bank integration on transactions (for example: debtorName and creditorName)

• not all fields are visible in Lunch Money (this is currently only supported for Plaid)

How do we ensure that the Payee field can utilize multiple fields?

Initial thoughts:

1. Allow the user to choose a fallback option to other fields (what happens if those are empty?)

2. Highlight that some fields don’t exist on all transactions (there’d be an edgecase where all historic transactions at the time of import have it, but then new transactions don’t)