Prevent scrolling the background when a modal is open

[DanReyLop]

To test:

• Go to any page on Calypso that lets you open a modal. For example, go to your site's Settings and change the site language.
• Try to scroll using your mouse/trackpad while having your mouse pointer outside of the modal.

What happened before:

• The background page scrolls.

What should happen:

• The user can't interact with the background page in any way, not even scrolling.

This PR blocks background scrolling in all the modals in Calypso. I think that's the most sensible behaviour, since a modal dialog should prevent the user from interacting with the background entirely, and scrolling the background can be distracting. Can you folks think on any scenario when we would want the background to be scrollable?

[matticbot]

Test live: https://calypso.live/?branch=fix/prevent-modal-background-scroll

[blowery]

Thanks @DanReyLop!

[blowery]

I think this is a sensible default. If we end up with a section that really wants background scrolling for some reason, that section could override the no-scrolling rule and enable it. I can't think of any reason to do that though, unless we're using Modal in a really weird way.

[alisterscott]

Is this one good to go?

[DanReyLop]

I was waiting for a and then forgot about it.

@blowery are you OK with this change? I'm hesitant to go ahead since I don't know all the implications of changing all of Calypso's modals behaviours.

[blowery]

@DanReyLop I'm ok with it. If we find places where we need background scrolling to work, I'd be surprised, but we can also fix those cases.

[blowery]

LGTM