Show different datepicker for Single Day, Multi-day, and No Date events (eg online only)

[summasmiff]

Goals of this PR

• Show separate datepickers for different event types
• Refactor SubmissionForm & child components into more modular components for easier state management and future improvements

New datepickers

New datepicker states are based on these Figma mockups.

- No Date (Online Resource)

Hides the picker completely.

- Single Date (Single-day event, Call for Entry)

Renders one DateTimeVenueEditor.

- Multi-Day (Gallery Show, Multi-day Event, Other)

Renders one DateTimeVenueEditor. Once a first date is selected, shows a button to add more.

New "Add Venue" behavior

The "Add Venue" form is now less prominent to prevent unnecessary venue additions. It is available from the Search Venues component as the final option, "Add a new venue...". When clicked, this opens a modal.

Refactor notes: new components: EventDateTimesVenuesEditor & DateTimeVenueEditor

The purpose of this PR is to add better UX for different types of events. Adding a bunch of new state management into the already-long SubmissionForm felt like a step backwards. Instead, this PR adds a couple new components to manage the local concern of "which datepicker to show".

EventDateTimesVenuesEditor

The main smart container managing the DateTimeVenues array (v-model="calendar_event.date_times"). It handles the conditional logic for three states, Single Day, Multi-Day, and No Date.

DateTimeVenueEditor

A 1:1 dumb presentation component combining the time and venue pickers. Manages confirmation/edit states and toggles between editable or static text displays.

Updates have been made to DateTimePicker and VenuePicker to make them "dumber" (more composable).