Limit number of months shown on What's On event calendar

[LaurenFBaily]

What?

Limit the current number of months showing on the event calendar to the next 4 months (including current month).

Why?

Our current process is to add events to the site quarterly (maximum 4 months ahead), however relaxed openings for exhibitions which have occurrences much further ahead than the rest of the programme. This means the What's On calendar is displaying several months in the future, some with just one event and some with none which is confusing for users.
Users can click the 'All events' button below to see all events or events further in the future.

Background

This is a different solution to the same problem documented here: #10434

Current logic is to fetch 100 events and every month is displayed up to the furthest away date.

Done when

• [ ]
• Documentation updated