handles thumbnail usage in views to replicate sharp 8

[PatrickePatate]

In Sharp 8, you could use thumbnail() without parameters in blade templates and sharp would render it as a string (thumbnail url).

Now with Sharp 9, thumbnail() is fluent and can be chained with other methods (like addModifier()).
When using ->thumbnail() without parameters, sharp now returns a Thumbnail model.

This PR tend to replicate previous comportment with fluent Thumbnails, by implementing a __toString() Method in Thumbnail model that executes ->make().