Bazarr Arbitrary file read in /system/backup/download/ endpoint

Bazarr manages and downloads subtitles. Prior to 1.3.1, Bazarr contains an arbitrary file read in /system/backup/download/ endpoint in bazarr/app/ui.py does not validate the user-controlled filename variable and uses it in the send_file function, which leads to an arbitrary file read on the system. This issue is fixed in version 1.3.1.