Take advantage of Yarn's preservation of links while installing and set up links
first and install second. This should result in a small speedup since we no
longer install the published SDKs just to replace them on the next line.
Most `npm` operations are replaced with `yarn`, which generally has better
behavior. However, steps like publish that write to the NPM registry are left to
`npm`, which currently handles these tasks best.
Hopefully this will end up simpler than having to figure out in
riot-web what the relative path is from react-sdk's
src/languageHandler.js to riot-web's webapp directory.
