The opt in link is part of the sidebar in the current modern version
of vector. The legacy sidebar has been split out and frozen. For this
reason I think it makes sense to move the styles for the button into
the sidebar stylesheet.
The class name is renamed to reflect the hierarchy. We are not using
BEM but we are using hyphens to describe location so the newly proposed
names `mw-sidebar` and `mw-sidebar-action` seem more applicable than
the previous generic class names.
This doesn't impact caching given anonymous users cannot see the opt
in/out link.
Change-Id: I2991e941592a0678f40c73bb6daeab71ada6b7c3
d49eb1e0ff