Redirect /firefox/update to /firefox/new. Bug 929775.

- Remove /firefox/update from urls.py.

- Remove /firefox/update template/asset files.

- Replace firefox.update URL refs in other templates with
  firefox.new URL.
Merge pull request #1367 from jpetto/bug-929775-redirect-fxupdate-to-fxnew

Redirect /firefox/update to /firefox/new. Bug 929775.