Hide widget replacement toggle

[ghostwords]

The first part of #3007. We could make the deprecation more visible by doing something like #2783, but it doesn't feel like we need to in this case.

[lenacohen]

LGTM! This is more of a comment on this issue than this PR, but I think that if we're making the Widget Exceptions work as expected, we should include shortcut for people to allow all widgets globally. This could be a button that adds all widgets to the exceptions field, or another checkbox.

[ghostwords]

Thanks! Why would we want to make it easy to inadvertently lower your privacy across the board?

[lenacohen]

Thanks! Why would we want to make it easy to inadvertently lower your privacy across the board?

We'd definitely want to include text warning that "Allowing all widgets will enable tracking by..." If people are using the "Enable widget replacement" toggle, I imagine that means there are people who don't want to click to show widgets on every site. But I think it's okay if we just allow users to enable widgets selectively, then people who don't like the feature can manually select every widget.