[Enhancement] Capture playlist_index for use with output templates

[kieraneglin]

What's new?

• Captures playlist_index when first indexing for use in output templates
  • Does not update - remains locked in when the record is first created
    • Due to this, there is no way to retroactively update existing media items. This only works for media items going forward
  • Not compatible with channels, playlists that update often, or fast indexing. This is an intrinsic shortcoming of changing the order of a defined collection so these compat issues are expected and will not be addressed
  • Essentially, it's just for providing a way to order static playlists and does not have broad utility beyond that
  • Related to [FR] Playlist episode naming for better media center organization. #302, [FR] need a way to order by playlist order rather than upload order #309

What's changed?

N/A

What's fixed?

N/A

Any other comments?

N/A