Fix gifv expandos overlapping content and double-size image expandos

Related to #1672.

The existing debounce did nothing, it just set the placeholder a bit later: to the height of the closed expando (0 height) instead of the opened, unloaded expando (~0 height).

This works because we already have code to update the placeholder size when revealing expandos (in `revealImage()`), but only when the expando button stores the `associatedImage` in jquery data. Previously, this was only done for image expandos.