[6.x] Fix asset editor prev/next buttons and "Edit Image" button in Bard

[joshuablum]

The previous/next buttons in the asset editor were broken in two places. This PR fixes this and also does two other small things:

1. Fix prev/next buttons in the editor not doing anything when opened from an asset fieldtype. The editor emits both previous and next but AssetTile and AssetRow weren't listening to those. They now cycle through the assets selected in the field.
2. The prev/next buttons showed even when there was nothing to navigate as in only one image/asset there. Like with an asset fieldtype configured to max_files: 1 and images used in Bard. Now hides the buttons when there's just one asset.
3. A fix for Bard's "Edit Image" button not working. A regressions accidentally introduced by [6.x] Relationship UI adjustments #14719.