Remove soft-delete mechanism.

Summary:
We weren't using it for any real purpose, and it tended just to complicate
queries and reasoning about relationships. In particular, drafts may be
soft-deleted via the API, which can cause major problems during IMAP sync
(where soft deletes are ignored).

* Syncing back deleted objects (drafts and events, currently) has to work
  without access to the object itself (because the object is deleted for real).
* Some relationships now require `passive_deletes=True` to be configured in
  order for database-level delete cascades to work properly.
* `session_scope()` and `InboxSession()` still accept an `ignore_soft_deletes`
  argument so that client code doesn't need to be updated immediately. This
  argument does nothing, and can be removed once all client code is updated.

Depends on D881.

Test Plan:
Run unit tests; should run on staging for a bit to shake out any
lingering issues.

Reviewers: kav-ya, spang

Reviewed By: kav-ya, spang

Differential Revision: https://review.inboxapp.com/D882