Rewrite "Using client-side modules…" React tutorial

[benjamn]

This section, specifically.

Once https://github.com/meteor/react-packages/pull/151 gets merged, the meteorhacks:npm and cosmos:browserify packages will no longer be necessary (when using Meteor 1.3).

[stubailo]

Actually we can get rid of like 70% of that content now:

• JSX transpilation should just be merged into whatever docs we have for ES2015
• Tutorials is now a tutorial on Meteor.com
• The NPM section will be outdated

We should eventually find a better home for this content that looks more "official". Perhaps some kind of "react" appendix to the docs or guide?

[benjamn]

Filed a task for this over at the react-packages repository: https://github.com/meteor/react-packages/issues/152