Rename __clientTemp and separate client and server build

[jhnns]

Following up #794

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Currently, all webpack output is located under __clientTemp. To me, this is confusing because the folder does not only contain client stuff, it's also not temporary. It's the actual build artifact that people will deploy.

I would recommend to rename __clientTemp to build or dist (we can bikeshed about that 😉) which contains two folders, client and server.

[doug-wade]

I dig it. If I understand #791 correctly, __clientTemp is a particularly bad name, since it will also be the server temp directory. I'm leaning towards build, personally, though either strikes me as an improvement.

[breezewish]

Could you also make it configurable? Currently we have deployment phase and dev phase. Currently they use the same directory and affect each other.