Find WhatsApp thumbnails by (long) path

[wladimirleite]

Working with iOS extractions, I noticed that some absent WhatsApp media also do not have thumbnails in the HTML generated by the internal parser.
However, some of them do have thumbs in the external parser view.

The same method used to link medias described in #2044 can be applied to thumbs (i.e. thumb path is stored in the SQLite and the thumb file is present in the referenced path).

To avoid conflicts, I will work on this after #2401 and #2431 are merged.