md-dialog should block page scroll

[improvise]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

When a dialog is open, the page should not scroll. Only the dialog should be scrollable.

What is the current behavior?

When a dialog is open, the user can still scroll the page.

What are the steps to reproduce?

https://plnkr.co/edit/70UnwmTG2mvLWkhOSU4a?p=preview

What is the use-case or motivation for changing an existing behavior?

When a dialog is open, a scrollable page in the background is confusing for the user.

In Angular Material 1, this is configurable with disableParentScroll https://material.angularjs.org/latest/api/service/$mdDialog . While that would be nice to have, disabling the behavior would work as well.

Which versions of Angular, Material, OS, browsers are affected?

I haven't tested extensively, but this affects at least the latest version of Angular / Material / Chrome that we're using.

[fxck]

dupe of about 10 other issues.. also #1971

[jelbourn]

Now tracking via #4093

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