Let's decide on how best to represent MediaWiki galleries in the media endpoint output.

Strawman: include a gallery_id property for each item in a gallery, assigning it the value of the Parsoid-assigned id attribute for the gallery root.

Con: We are currently stripping most if not all id attributes from elements in the in the page endpoint responses, so we might have to update that treatment depending on the use case. On the other hand, the client could simply use gallery_id as an arbitrary identifier for grouping items together, in which case the HTML id stripping wouldn't matter.