Refactor to single `budget` object structure

Changed

• Change budgets array to single budget object

Added

• Enable editing a category

Add accounts

Added

• Add accounts data store
• Enable adding a (financial) account
• Enable seeing the list of accounts

Fixed

• Extract character-related functions to a helper file

Improve UX for specifying amounts

Fixed

• Improve UX when specifying the budgeted amount for a category

Fix paths for GitHub Pages hosting

Fixed:

• Put index.html in root to fix GitHub Pages hosting
• Make asset paths in index.html relative so they work on GitHub Pages hosting

Add categories

Added:

• Add basic application structure
• Enable adding categories
• Save categories to localStorage
• Load categories from localStorage (if found)