Add JSON attribute for "thumbnailImage"

[pbrenton]

As an aid to in-form species identification it would be useful to have a JSON attribute for a thumbnail image which can be placed in the form for single species surveys and inside table rows where species are listed in the table, especially as fixed rows.

This should retrieve the ALA preferred image for the species unless an over-ride to a fixed URL is specified in the JSON as a dependent.

[matildastevenson]

@pbrenton, can you confirm this functionality exists with the speciesSearchWithImagePreview?