Fix event edit form dropping organisers on save

[artfuldev]

Fix #2868

Keep an event's existing organisers when editing it, instead of silently clearing all of them on save.

Why did we do this?

The organisers multi-select on the event edit form never showed existing organisers as selected, even though they were still assigned. Anyone who saved the form without re-picking organisers submitted an empty selection, and the controller dutifully revoked every one of them.

Two more commits fix test correctness issues this change surfaced or depends on:

• The "adding all chapters" feature test asserted on .search-choice without scoping it to the chapters widget. That class is shared by every multi-select on the page, so once organisers render correctly (this fix), the assertion started counting the organiser's chip too.
• The today_and_upcoming scope spec froze time at the real wall-clock instant and subtracted an hour to simulate "happening now" — if the real run time was within an hour after midnight, that pushed the workshop into the previous day, dropping it from a scope that filters from start-of-day. Unrelated to the organisers fix, but pinned it to noon so it can't cross midnight.

Will this break anything?

No. The organisers fix only corrects pluck(&:id) to pluck(:id), which is what the selected: option already expected. The two spec changes only make existing assertions more precise; they don't change any application behavior.

How was this tested?

Organisers fix — captioned screen recordings of the actual edit → save flow against a real admin session, showing the event page (Team section) before editing, the organisers field mid-edit, and the event page again after saving:

Before fix — the organiser is genuinely assigned (Team section shows them), the edit page shows the field with 0 chips selected, and saving without touching it wipes them — the Team section now says "This event has no organisers.":

before-fix.webm

After fix — the organiser shows as a selected chip on the edit page and survives a save — the Team section still shows them afterward:

after-fix.webm

today_and_upcoming flake — deterministic reproduction against the real scope and database, travelling to a fixed post-midnight instant (00:15) — real recorded terminal session (animated):

.search-choice scoping — visible in the after-fix video above: the organiser's chip renders inside #event_organisers_chosen, a sibling widget to #event_chapter_ids_chosen, which is exactly why the chapters test's unscoped selector needed narrowing.