Feature: Set frequency unit by query string

A feature request that would enable the setting of frequency unit in the URL like:

• https://example.org/donate/?is_recur_radio=month
• https://example.org/donate/?is_recur_radio=year

This would override the "Select recurring donations by default?" setting.

This would allow the combination of frequency unit and price set option in the query string which is useful in email campaigns or other landing pages:

• https://example.org/donate/?is_recur_radio=month&price_123=XX.