WIP: Add Budget Support and Update Style

[treed593]

This assumes that #55 has been merged. This PR will do the following

• Add Tabler for additional Styles/JS
• Rename .j2 files to .html for IDE compatibility and readability
• Add the ability to view a list of Budgets from a Book
• Add the ability to view the details of a Budget from a book
• Add additional Jinja filters
  • currency - This filter will use babel to format an integer as a specified currency, including negative number support
  • flip_sign - This filter is used to convert negative numbers to positive, and vice versa. This is useful when calculating Loss/Profit and displaying the appropriate sign on the currency.

To-Do:

• [ ] Update style throughout the app to be consistent with the new Tabler styles