cubicweb #2093183 extractSortValue() js method should fallback on cell content [validation pending]

When a colrenderer declares a column sortable, the cubicweb.sortValueExtraction() is used as the sort key function. This function looks for a cubicweb:sortvalue attribute in the corresponding td. If the attribute is not found, the fallback sort key is an empty string. It should be the cell text content instead.

done in3.15.0
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closed by#2dedcc15208d [table] use cell text content when cubicweb:sortvalue is not defined (closes #2093183)