Add a 'fortnightly' frequency for budgets

[JP-Ellis]

Summary

I would love to see a fortnightly time interval for use within budgets.

Motivation

In Australia, it is quite common to have income and debits charged at a fortnightly (every 2 weeks) frequency. It would be really convenient if the budgets in Fava could support a fortnightly interval:

2024-07-01 custom "budget" Expenses:Health:Gym    "fortnightly" 29.00 AUD

Alternatives

Currently, I just do the conversion explicitly into a weekly amount, but it isn't quite as nice:

2024-07-01 custom "budget" Expenses:Health:Gym    "weekly" (29.00 / 2) AUD

Implementation

I would be happy to help contribute a PR if that is a change that would be welcome.