Allow client to remain logged in when 3rd party cookies are blocked

This comes out of a user question in https://github.com/hypothesis/product-backlog/issues/872 - Why does Chrome extension log out on each page load if 3rd party cookies are not allowed by default?

Answer:

> Third-party cookie blocking typically prevents third-party iframes (ie. those with a different origin than the tab itself) from storing data in local storage as well, because otherwise that would be used to bypass cookie blocking. The end result is that the client typically cannot persist the login if third-party cookies are blocked... We probably could make use of extension-specific APIs to avoid this problem. I think Nick suggested doing that aeons ago.

Logging this as a feature request so we can track if/when other users start to ask about this.