cubicweb-blog #2551626 provide sitemap rules if cube seo is loaded [validation pending]

and put cube seo in recommends

prioritynormal
typeenhancement
done in1.10.0
load0.100
load left0.000
closed by#268148de1a95 [views] refactor a bit the sitemap rules and make seo a recommend (closes #2551626), #26c0ad70ac58 [view] add support for sitemap generation (closes #2551626)
patch[views] provide sitemap rules if cube seo is loaded (closes #2551626) [applied][view] add support for sitemap generation (closes #2551626) [applied]