Feature: Subcategories

[theperu]

🙋🏻‍♂️ Add subcategories

📋 Description

Summary:
We should allow a user to track their expenses also with subcategories. Ex. Category: groceries, Subcategory: Coop

Expected Behavior:
From the settings or the new transaction page the user should be able to add subcategories to an existing category, and he should be able to use it in the new transactions.

Notes:
Lets consider how to handle them both in the frontend (what data do we want to show about these?) and DB (how do we want to store them?)