Move all grammars that use Git repos to submodules

This makes it so we don't have to redownload all the grammars every time we build the grammars gem. We test that the list of submodules in the repo matches the list in grammars.yml. Eventually we'll be able to test that the list of scopes for each submodule is accurate as well, which was the original motivation for this change.

/cc @bkeepers @arfon @pchaigno 
I'd love to find Git mirrors of these repositories, or find Git-hosted grammars to replace these. Then we can get rid of the download logic entirely and building the grammars gem won't require a network connection.