Updates `categorize_data` to work with the `Transaction` class, which is now
used by the database.
Vacation categorizing will no longer replace previous category assigned.
`DBManager` gained new method to retrieve transactions between two dates
without any category.
Add method to `Transaction` to prepare for category update in database.
Transaction's returns from database queries are checked and assigned in
`if`s with the walrus (`:=`) operator.
Luís Murta
d8c9f3f9b9
