Update sibling/burying behaviour to be in line with Anki 2.0.14.

Includes changes in these commits:
rework sibling handling and change bury semantics (afde116)
unbury on open for now (a2463eb)
unbury on open/close and make sure we set usn when unburying (0bef1d8)

Changes that expose burying options in the UI are not included here.
 we can remove graves when uploading and save space (dae/anki e9ab912)