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