This commit is a partial revert of 4628b26, affecting standalone
annotation pages only.

That commit changed the AnnotationViewerController (the component
responsible for standalone annotation pages) to make a single request to
the search API to load an annotation and all its replies, rather than
two requests -- for the annotation and then for its replies.

This works just fine so long as the top-level annotation is not itself a
reply. In that case, however, the search endpoint returns nothing, as
the (temporary) `_separate_replies` parameter ensures that only
top-level annotations are matched.

This commit switches back to making two requests to populate the
client-side data structures for the standalone annotation page:

1. To fetch the annotation referenced in the URL (which may itself be a
   reply).
2. To fetch any replies to the annotation referenced in the URL.

Fixes #2775.