R1:3963c86ad5e5

Pass timeline view data to comment previews, restoring Differential comment previews

Summary:
Ref T13222. In D19918, I refactored how timelines get "view data". Today, this is always additional data about which images/changesets/diffs are visible on the current revision/commit/mock, so we can tell if inline comments should be linked to a `#anchor` on the same page (if the inline is rendered there somewhere) or to a `/D123?id=1&vs=2` full link on a different page (if it isn't), but in general this could be…