Switch from HTML main element to body

It is my feeling that `app.layout` should be the root of the application. I think using `<body>` instead of the `<main>` element is more seamless and it doesn't come with any constraint (as to how your CSS, your markup, and if the needs to provide a shim for old IE).

At the same time, I also removed the bbb-layoutmanager adapter modules (which was needed to send this PR on BBB repo).