Makefile: make first run of `make install` init/update submodules
Makefile: make first run of `make install` init/update submodules

Summary:
Adds some logic to the Makefile so that the first run of
`make install` will also run `git submodule update --init`,
thereby simplifying the installation and hopefully making
differences between submodules and node_modules less annoying.

You can now just clone the repo and run `make server` and
get a working server.

Fixes #17

Test Plan:
I made a fresh clone of the repo, changed this file, and
ran `make install` then `make server` and verified that
submodule pulling happened like I expected.

I should have tested a raw `make server` from a fresh clone,
but I'll do that once I've actually pushed this change to
the repo so that I get it when i pull the repo.

Auditors: alex

abe4mvp/perseus,alexristich/perseus,ariabuckles/perseus,iamchenxin/perseus,learningequality/perseus,sachgits/perseus,iamchenxin/perseus,abe4mvp/perseus,chirilo/perseus,daukantas/perseus,chirilo/perseus,sachgits/perseus,learningequality/perseus,alexristich/perseus,ariabuckles/perseus,daukantas/perseus,ariabuckles/perseus,ariabuckles/perseus