Build clamshell and blip in runservers

A set of changes to adapt `runservers` to new webpack workflow for Blip and Clamshell.

Basically we build both apps with a "local" config, then launch their static servers.

Since the build script is quite verbose, I redirect its output to `/dev/null`.

Since the build can take some time, I added to ability to set `export SKIP_BUILD=true` before running the `source tools/runservers`. But this is only "if you know what you are doing", i.e. by default it will always build.

@cheddar @kentquirk let me know what you think?

Thanks!

See https://github.com/tidepool-org/tools/issues/18
`develop.js` doesn't exist anymore (it was a server that rebuilt the whole app everytime you hit the browser refresh).

It is now replaced with `webpack-dev-server` (run via `npm start`), which is a lot more "developer-friendly", i.e. it rebuilds only parts of the app as you work on it. However, I don't think it's great for something you want to run in the background (it's better to run in your dedicated terminal when working on the app).

That's why I offered the solution for `runservers` to build the app once, and then start a static file server in the background.

Does that make sense?