Move merged-share-backend to webpack

For #13608

Since we have webpack anyway it make sense to let webpack do the
bundling instead of ourselfs. This leads to minified code (so less
transfer). And the webserver can just handle the request.

As a bonus we get a map file so debugging is easier than with our
JSCombiner stuff.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>