00:00:00 to midnight conversions

[stevegrunwell]

In places where intervals either start or end at midnight (e.g. a daily interval or an hourly interval that started at 11:00pm and will end at 12:00am), it might be nice to automatically convert these values to user-friendly strings like "midnight".

For example: "Please check back after midnight" v. "Please check back after 12:00am".

The same reasoning might also apply to "noon" v. "12:00pm".

If we move forward with this, we should also make sure that the Admin::admin_notice() method is updated to match.