scheduled reminders: date field confusing UI

The scheduled reminders date field ("When" field) is confusing and easily misinterpreted. The intended behavior is to allow you to either schedule the reminder on a specific date OR schedule based on a relative date. However, there are a few issues:

1. the UI could be interpreted as mutually cooperative rather than mutually exclusive. you might set a specific date AND a relative date, thinking that the reminder will go out whenever the first condition is hit (or perhaps multiple times, when each condition is hit)
2. if you select a specific date and have already set a relative date, the relative date values are deleted on save. the specific date effectively has precedence over the relative date (preserving the mutually exclusive nature of the options). but that is not clear to the user and isn't know unless they re-open the reminder and notice the relative date values have been cleared

We should add a radio-button toggle field to switch between the two options and show/hide the appropriate fields, clearing values for the hidden option when the toggle is flipped.