unbury in deckbrowser, not deckduelist() (#946)
rework sibling handling and change bury semantics

First, burying changes:

- unburying now happens on day rollover, or when manually unburying from
  overview screen

- burying is not performed when returning to deck list, or when closing
  collection, so burying now must mark cards as modified to ensure sync
  consistent

- because they're no longer temporary to a session, make sure we exclude them
  in filtered decks in -is:suspended

Sibling spacing changes:

- core behaviour now based on automatically burying related cards when we
  answer a card

- applies to reviews, optionally to new cards, and never to cards in the
  learning queue (partly because we can't suspend/bury cards in that queue at
  the moment)

- this means spacing works consistently in filtered decks now, works on
  reviews even when user is late to review, and provides better separation of
  new cards

- if burying new cards disabled, we just discard them from the current queue.
  an option to set due=ord*space+due would be nicer, but would require
  changing a lot of code and is more appropriate for a future major version
  change. discarding from queue suffers from the same issue as the new card
  cycling in that queue rebuilds may cause cards to be shown close together,
  so the default burying behaviour is preferable

- refer to them as 'related cards' rather than 'siblings'

These changes don't require any changes to the database format, so they
should hopefully coexist with older clients without issue.