Add HTML meta specification to link to hosted SCM instances

[rasky]

To simplify crawling of hosted SCM instances owned by public administrations, we could come up with something like:

   <meta name="publiccode-server" content="https://gitlab.administration.gov"
               data-server-type="gitlab" />

By putting this into the homepages of administration's sites, we can simplify crawling. For instance, in Italy, we do have a (partial...) list that we would crawl searching for those links.

[ruphy]

An alternative to the meta tag would be to use RDFa: http://rdfa.info/

This way we could embed additional information about the PA itself, or encode additional metadata (e.g. where the source code of the website itself can be accessed).

[ruphy]

@rasky did we decide to skip this in the end?