Changed submodule source from rw to read-only repo so anyone can build website

- Originally, we were using the read-only repository to get our
  submodules. This caused problems for people without read-write access.
- By changing to the read-only repository, anyone can now fully make the
  website, regardless of their permissions.

Signed-off-by: Anjandev Momi <anjan@momi.ca>