dev/core#2535 Scheduled reminders: Add error if both absolute and relative date entered

[larssandergreen]

Overview

Prevents users from saving a scheduled reminder that has both an absolute and relative date set, resulting in the relative date being ignored.

Before

This can be submitted successfully, though only the absolute date is actually saved, potentially creating confusion.

After

The form cannot be submitted if both relative and absolutes dates are filled in.

[civibot]

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