WIP: Make web ui build robust and reproducible.

[jackgr]

This PR contains only the build script portions of #8776. It is part of the UI cleanup requested by #7122.

It does the following:

• Adds hack/www/build-www.sh to invoke the www directory build.
• Adds a node based container based to make the www directory build reproducible.
• Adds build/www/run.sh to run hack/www/build-www.sh in the container.
• Moves the generation of pkg/ui/datafile.go from the www directory build output into library functions.
• Makes hack/build-cross.sh and hack/build-go.sh call the library functions, so that the output of the www directory build is picked up by the go build.
• Makes build/release.sh build the whole UI, including the www directory and pkg/ui/datafile.go.
• Adds lines to .travis.yml and shippable.yml to make them build the whole UI.

This PR is marked as WIP because it contains changes to the go build processes. While care has been taken to follow the "gold standard" of shell code in every modification, except those that were straight copies of existing code, errors in this PR could break the build.

cc @bgrant0607, @thockin, @bcbroussard, @preillyme

[k8s-bot]

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[bcbroussard]

@jackgr - Just a heads up that #8639 and #8659 will change the travis and shippable configs a bit

[k8s-bot]

Can one of the admins verify that this patch is reasonable to test? (reply "ok to test", or if you trust the user, reply "add to whitelist")

If this message is too spammy, please complain to ixdy.

[j3ffml]

Assigning to @bgrant0607 for milestone triage.

[jackgr]

When #10077, the UI will no longer be built in the main repo. This PR will then no longer be needed.