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
Yeah, that's fine.  Fwiw, even when I do backend development, I generally start up everything, kill what I'm working on and start them up manually.

This just means I'll do that for blip/clamshell as well.