Re-added update-extern script.
Squashed 'extern/frontend/' content from commit 147965a

git-subtree-dir: extern/frontend
git-subtree-split: 147965aa170984e301246946011eaffa25ba230b
Merge commit '52b6e15cb3ea1e644529872ea2f97f1be57676b9' as 'extern/frontend'
Partially use shared head and footer