General: Integrate thumbnail looks for thumbnail to multiple places

[iLLiCiTiT]

Brief description

Integrate thumbnail can look to more places where thumbnail for instance could be found.

Description

Before this PR was thumbnail found only on published representations or from context thumbnail. There are 2 missing sources: First is at key "thumbnailPath" on instance data and second is on not integrated representations. Added these 2 options.

Order of thumbnail path resolving

1. Look for published representation with name "thumbnail"
2. Look for not published representation with name "thumbnail"
3. Look into instance data for key "thumbnailPath"
   If any step has available value but the path is not valid then go to next step until hit end.

Testing notes:

1. Thumbnails are integrated even if it's representation is not