Refactoring recurring transactions

[mikev-cw]

This aims to address #144. I focused on Backend operations only.

Removed recurring_transaction_amount model, and added recurring_transaction.
Now we've a table that list recurring transactions. When the app is opened, not more than once a day thanks to shared preferences, system will check if a transaction (or more) is needed, and adds them to the transaction table, based on their recurrency type.

Also changed the behavior of "clear db" button on settings: now it re-generates all tables. This is useful when we edit models: there will no more need to delete database from OS.
To check this PR, just chekout, run, press "reset DB" and fill with demo data.

I'm leaving this in draft for now: i need to go deeper on some behaviors, update tests, and remove some dev debug code.
Meanwhile feel free to share your opinions if you want.
Also if anyone wants to help with frontend, which I am not very familiar with, it would help.

[K-w-e]

I can help with the frontend changes.
I'll work on the recurring transactions display section.

[mikev-cw]

@K-w-e Thank you, it would be great!
There's also how the add_page.dart reads and writes recurring data. Not checked yet how that part needs to be implemented.
Reach me any time if you need help, i plan to open this PR for review within this week.