[marameref]

This adds support for extending an existing scheduled talk or page into additional slots through the drag-and-drop editor.

Dragging a scheduled cell into an empty cell in the same venue adds the target slot while preserving the existing placements. Clicking × removes only that placement; removing the final placement deletes the schedule item.

The changes also:

• Enable dragging scheduled cells and newly placed items.
• Fix the missing IDs on entries in “All Talks”.
• Give schedule cells and delete buttons unique IDs.
• Reject extensions into occupied cells or another venue.
• Add regression tests for adjacent and non-contiguous extensions, slot removal and immediate extension after placement.

Validation:

• 20 Chrome schedule-editor tests passed.
• 84 non-browser schedule tests passed.
• JavaScript syntax and Git whitespace checks passed.

Related Debian issue: https://salsa.debian.org/debconf-team/public/websites/wafer-debconf/-/issues/75