Verify language packs after downloading them when building with Gitian
We should verify that we actually get the language packs we really want to get when running the fetch-inputs script. That serves two purposes: 1) it is a defense in depth against attackers that may be able to modify the contents of the language pack but don't own Mozilla's signing key 2) We may avoid problems with old language packs that somehow slipped in just before starting a new TBB build.