Allow a user to sort facets

Refs https://gitlab.com/teklia/requests/-/issues/782

A search user should be able to sort any facets :

1. as currently by nb of occurences in results
2. by facet name

I suggest adding a simple "button" displayed as a link on each facet header : "Sort by occurences" and "Sort by name", toggling between both modes. Only the link toward the non-displayed mode is displayed.

By default we'll sort by name.