cubicweb-seo #3376763 Disable sitemap by default [in-progress]

An empty sitemap is worse than no sitemap.

Currently, with only the seo cube enabled, the sitemap may be empty if the other cubes are not "SEO-enabled".

So, the default behavior should be to enable robots.txt (which is always a good thing), but make sitemap something that is willingly enabled.

done in0.3.0
load left1.000
closed by<not specified>
patch[robotstxt] Make a separate view for sitemap-enabled robots.txt [in-progress]