Apply strict month & year selection limits.

[Milesjpool]

Currently months and years may be selected outside of the provided min-date/max-date range. This makes for bad UX as, at the high-level, a user cannot see the extent of the valid range.

This PR adds a behavour.date.strictDateLimits option, which applies the min/max date selection limits more strictly, at the month and year levels.

Limitations:

• this only considers min/max limits and does not account for specifically disabled or enabled dates. A month could still be selected where all days are disabled.
• Empty months & years can still be selected, e.g. by navigating horizontally through months, or changing the year with an invalid month selected.