[WIP] Restore feature to scroll for annotations

Clicking on an annotation card will bring (one of the) highlights belonging to that annotation into the view in the document.
> Is this check necessary? When would it ever not be an Object?

I have no idea, but the `activate()` function, [a few lines above](https://github.com/hypothesis/h/blob/c1b8019dc3136096783ac3fa8e91bb54df0417fe/h/static/scripts/controllers.coffee#L338), which is [called](https://github.com/hypothesis/h/pull/1720/files#diff-22eac557e789ec06ce4836e330e8e943R41) when the mouse hovers over the annotation card, has the same check.

I don't know how added it or why (unfortunately there is no comment explaining it), but I am pretty sure that the test is there for a reason, so I am unwilling to drop it.

Here are a few situations, from the top of my head, that *might* (or might not) cause the problem this checks is protecting us from:
 - The annotation is new, have not been saved yet. (Maybe it has not yet crossed the bridge, has not tag, etc.)
 - We are currently loading / unloading the annotations, and the app is in a transient stage.
 - We are using the annotation card in a different context (standalone page, stream, whatever), where the annotation object is not available via the same channels
 - ... whatever exceptional cases I am currently forgetting

So I would like to keep that check, just to be on the safe side.