HOLD: Make event start/end time optional with all-day rendering

[maebeale]

What is the goal of this PR and why is this important?

• Event start/end time was required even when only the date matters, forcing admins to invent a time.
• This makes the time optional: a blank time renders the event as all-day (no time shown) everywhere it appears.

How did you approach the change?

• Dropped required from the start/end time inputs; a blank time persists as midnight and is treated as "no time" via a new Event#all_day?.
• Centralized the all-day handling in EventDecorator#times, ApplicationHelper#event_times_label, and calendar_links (which now emits proper all-day calendar entries), so the show page, cards, tickets, mailers, and registration page all follow.
• Styled the optional fields (start/end time, registration-close box) grey instead of white and added a hint explaining the all-day behavior.

UI Testing Checklist

• On event edit, leaving a start/end time blank saves and the event shows as all-day (no time) on the show page, cards, and registration page.
• Entering a time still shows the time range as before.
• Calendar links (Google/Apple/Outlook/Yahoo) produce an all-day event when no time is set.
• Optional fields (time inputs, registration-close box) render grey; the all-day hint is visible.

Anything else to add?

• "No time" is stored as midnight in the viewer's time zone (no migration), so an event can't be set to start exactly at 12:00 AM. A boolean column would make this zone-proof if it ever matters.

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