[STTNHUB-379] fix: Planning items not shown in event preview with AGENDA_DEFAULT_FILTER_HIDE_PLANNING config

[MarkLark86]

Purpose

When AGENDA_DEFAULT_FILTER_HIDE_PLANNING config is enabled, no Planning item's are being displayed in the preview of an Event when in the "Events & Coverages" filter.

What has changed

• Make sure the back-end includes the planning_items attribute on the Event
• Expose the AGENDA_DEFAULT_FILTER_HIDE_PLANNING back-end config to the front-end
• If item type filter is Events & Coverages and AGENDA_DEFAULT_FILTER_HIDE_PLANNING config is on, then don't include extra dates from planning items for showing in the list (only show the top level Event).

Checklist

• This pull request is not adding new forms that use redux
• This pull request is adding missing TypeScript types to modified code segments where it's easy to do so with confidence
• This pull request is replacing lodash.get with optional chaining for modified code segments

Resolves: STTNHUB-379