feat(AccessKit Disable GIFs): Pause additional elements; fix misc position/hover issues

[marcustyphoon]

Description

This:

• Adds Disable GIFs pausing to trending tag card elements (on the current trending page, not the in-testing future one) and activity item preview images of both legacy and new ("generic") types.
• Fixes positioning of the sidebar house ad pause overlay (AccessKit Disable GIFs: Sidebar ad overlay misplaced #2317).
• Fixes hover-to-unpause on grid view posts (i.e. on the likes page) and trending tag card elements (on the in-testing/future trending page, not the current one).

Initially cherry-picked from #1862.

Let me know if you would like this split into stacked PRs.

Testing steps

• On an account with the current trending page, confirm that Disable GIFs pauses GIFs in a trending tag card, and that they unpause when the card is hovered. (At time of writing, there aren't any trending tags with GIFs, but I recall having tested this before, of course.)
• Like and comment on one of your own posts with a GIF. Confirm that the generated activity items have their images paused, and that they unpause on hover.
• Find a sufficiently racy post that the house sidebar ad for Tumblr Premium appears and confirm that it's paused correctly.
• Like a post with a GIF. View your likes page in grid view and confirm that hovering the square grid image unpauses the GIF. (It still has a grey overlay, so I can see the argument that this looks kind of odd. It also looks kind of odd for the GIF to not unpause, though.)
• On an account with the new trending page, confirm that Disable GIFs pauses GIFs in a trending tag card, and that they unpause when the post content is hovered.