Issue #2249723 by andrei.dincu: Add message in XmlSitemapForm if there are no contexts available
Issue #2249723 by andrei.dincu: Replace Category with Sitemap in XmlSitemapDeleteForm::submit()
Issue #2249723 by andrei.dincu: Create XmlSitemapRebuildForm class and update xmlsitemap.routing.yml
Issue #2249723 by andrei.dincu: Start XmlSitemapGenerator service
Issue #2249723 by andrei.dincu: Put all functions from xmlsitemap.generate.inc into XmlSitemapGenerator service