Basic sitemap handler

Summary: Ease bot indexing, so public project have the best visibility on the web.

Test Plan:

• • Go to https://c4science.ch/sitemap.xml, you should see the xml
• inspect this url with and without being logged, it should respect all policies
  • Wait a day for google to index the content and you should see better results

Reviewers: c4science

Differential Revision: https://c4science.ch/D58