Set event import window dynamically

[ivan-kocienski-gfsc]

Acceptance criteria

• [ ] events are imported regardless of date that site is built
• [ ] allEventsQuery method in src/Data/PlaceCal/Events.elm uses the current date to derive the window with which to limit events returned.

Implementation notes & questions

Consider using Pages.builtAt value see https://package.elm-lang.org/packages/dillonkearns/elm-pages/10.1.0/BackendTask-Time

Client has asked the window to be -3 months to +6 months (from build date).

Implementation plan

[katjam]

@ivan-kocienski-gfsc notes here for "not listing any events at all post 01-01-2024"