Allow importing of replies

**The problem**
1. Some instructors will create scaffolding that involves both annotations and replies to their own annotations, and they want a way to easily copy this scaffolding over to multiple groups within an assignment, and from course to course.
2. Students may wish to import class annotations to read them out of the LMS context within the (downloaded) document.

**The solution**
Selectively allow imports of replies alongside imported annotations, with the imported replies aligning properly with the imported annotations to match the source.

Start by reviewing the current annotation import flow and how replies are represented relative to their source annotations. Define the selective-import behavior and verify that imported replies remain aligned with their imported annotations in both copied scaffolding and downloaded documents.

Written by the indexing model from the issue text.