Refactor functions that created their own sessions when called from
within existing transactions, which caused "database table is locked"
errors in SQLite's shared-cache mode.
Changes:
- Add files.CreateWithSession() to reuse caller's session
- Refactor DeleteBackgroundFileIfExists() to accept session parameter
- Add variadic session parameter to notifications.Notify() and
Notifiable.ShouldNotify() interface
- Update all Notify callers (~17 sites) to pass their session through
- Use files.CreateWithSession in SaveBackgroundFile and NewAttachment
- Fix test code to commit sessions before assertions
49bba7f830