Features:
* Download attachments to cache folder (#181)
* Regularly delete attachments for deleted notifications (#142)

Bugs:
* IllegalStateException: Failed to build unique file (#177, thanks to @Fallenbagel for reporting)
* SQLiteConstraintException: Crash during UP registration (#185)
* Refresh preferences screen after settings import (#183, thanks to @cmeis for reporting)

Thanks:
* Many thanks to @cmeis, @Fallenbagel, @J117 and @rogeliodh for input on the new attachment logic, and for
  testing the release