Handle reversals for new imported transactions

[FestplattenSchnitzel]

Closes #700

[FestplattenSchnitzel]

Setting to draft b/c we should definitely add a test.

[lukasjuhrich]

Instead of doing that inline, we should provide an intermediate function which turns a MT940Transaction into a BankAccountActivity. For that, it needs from transaction.data the following fields:

• applicant_iban
• applicant_bin
• applicant_name
• purpose
• end_to_end_reference
• amount
• guessed_entry_date
• date

This should be a pure function

def activiity_from_transaction_data(
    transaction_data: TDataTypedDict,
    bank_account_id: int
) -> BankAccountActivity: ...

where TDataTypedDict captures the aforementioned keys as a TypedDict.
We can then use a fixed mt940 dump of test data to check whether all the cases are handled properly without even having to touch the DB.