Ability to play external subs and audio

Currently, when watching a video through the video player, the app can only detect subtitles containing the video file's name. (e.g. Bao.mp4 and Bao.srt or Bao1.srt)

This feature allows the user to select any subtitles from the Files application in iOS and add them as a permanent subtitle for that video file. It is compatible with both local files and remote files through servers.

Current Issue: Adding subtitle for a remote file currently works when the application is still active, even when the video player is closed, but do not last more than one lifecycle (when the application is closed and then re-opened, the subtitle is gone from the tableView).

LocalFile1

FilePersistence

Close #391 (closed) and #1033 (closed)