Make the category classification logic learn and improve over time

[brafdlog]

The simplest implementation for this is to fetch periodically from YNAB/Google sheets the mapping from transaction description to category and save this locally. Then use this for classifying future transactions with the same description.

An alternative implementation is to expose in the ui the transactions that are about to sent to the vendor and allow the user to classify them inside our app. This will mean we can store the mapping of description to category without fetching it from outside.

[baruchiro]

I think we made a decision, during the unification, to not saving the transactions as an application state. In my old app, I cut the importing phase from the exporting, in order to do some manual actions on the transactions.

I think it can be a good idea, but I think we need to close this issue and open it in the future.