Fee: auto-update directly from their Movement's AccountRows

When saving a Movement, now we automagically drop all related Fees, and we cleanly re-create all Fees, to keep them very consistent.

This avoids data duplication and weird inconsistencies.

Closes #28 (closed)

Test Plan:

Take a Movement and:

• save a single AccountRow with a Fee and a valid User ("saldo quota 2024"): a Fee is created on that year
• change the user: the Fee reflects the new user (the old Fee is nuked)
• save a single AccountRow without a Fee: its related Fee is nuked
• save two AccountRows with two Fees ("saldo quota 2099"; "saldo quota 2010"): two Fees are created on the specified years
• save it again, just changing the years in the raw notes: the fees are updated