cubicweb-conference #2727324 add cube seo to provide robots.txt and sitemap.xml [validation pending]

and include conference, track, talk in sitemap.

prioritynormal
typeenhancement
done in0.13.0
load0.100
load left0.000
closed by#4a632139bdd5 [views] add cube seo to generate robots.txt and sitemap.xml (closes #2727324)