Gerrit sanity bot should do a license check

Code should have correct license headers before being allowed into the product.

Since this can be quickly statically checked, ideally this job is done by the sanity bot attached to gerrit.

Most of the time, the problem is that people add new files without any license header at all.
Preventing that already would be good, so the sanity bot probably doesn't need to do a "full" license check - just applying some heuristics to detect if the new file has any license header (out of a set of known licenses) may be enough.