YNAB4 import: Add payee field

[acepin]

This is similar to #1008 OFX/CSV: Add payee field

Buckets now supports payees, with the completion of
#101 Add Payees (separate from memo) and #106 Auto complete payee/memo

Importing from YNAB4 should populate the "Payee" field from the YNAB4 data.
This would allow YNAB4 users to move their full transactions to Buckets.

Manually entering the payee for all the imported transactions isn't practical.
Especially for long-time users.
Some of them gave up on moving to Buckets because of this, as mentioned in #101.

This, and #491 are the main pieces needed for a smooth migration from YNAB4 to #Buckets.

Thank you.

[iffy]

Thank you for filing this. I'll aim to get this in the next release.

[iffy]

Included in v0.77.0 release (AUTOMATED COMMENT)