Add environment view header

This branch adds the new service/machine view header as see here: https://docs.google.com/a/canonical.com/file/d/0B5yaFXSrLF38aVk4ZXJZY0tJZWJTUy1GVXlScmZRS21rUFJv/edit

The contents of the header are still to be finalised and may well only contain the service/machine view links. As this is under a feature flag I figure we can finalise the contents in a follow-up branch.

The widget is being attached in app.js which won't be its final location, not sure if there is a better place for now.

When the machine view is implemented the sidebar won't be able to be minimised so the code to handle the header moving will go away.

The layout of the controls uses flexbox and I have tested that it works in Firefox, Chrome, IE10, IE11 and Safari. It would be good to QA that it works for others.