cubicweb #4644886 facetLoading div can be inserted several times [open]

The "#facetLoading" element is created and inserted in the DOM when the facet form is initialized but there's no safety belt that checks if the element already exists before creating it.

This can typically happen when the user clicks on a facet and the generated content also contain a facet form. In this case, this facet form is initialized once the new content is loaded and we end up with (at least) two #facetLoading elements in the DOM.

Besides not being clean, it is bogus since $('#facetLoading').hide() or $('#facetLoading').show() will not work as expected.

done in<not specified>
load left0.250
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