Make react-components a submodule

Summary:
Magicks!
Ran this script to automatically conver the imports:
find . | xargs -I% sed -i '' -e 's:"react-components/\([^.]*\)":"react-components/\1.jsx":' %

Test Plan:
Open test.html, see that it doesn't crash and that
webpack thinks the bundle is valid

Auditors: charlie, joel