Exclude orphaned replies from exports

[acelaya]

We are currently including orphaned replies when exporting annotations, and counting them in the amounts we display in the user selection dropdown.

This was okeyish for JSON, but is extra confusing for other formats, as we include annotations that are not displayed anywhere in the UI.

There's still a grey area, that we need to decide if, when exporting annotations from a single user, replies to other users annotations should be considered orphaned or not.