Don't require accepting third-party cookies to run the Web IDE

Problem to solve

The Web IDE requires you to enable third-party cookies for GitLab.com (or your GitLab domain) in order for some features to work properly. While GitLab doesn't actually use any third-party cookies, the Web IDE requires a service worker to run any webview (like those used to power the Markdown preview or in third-party extensions). The problem is that web browsers don't differentiate blocking service workers from third-party cookies. So you can't get the (safe) functionality of a service worker without accepting the (potentially unsafe) behavior of allowing third-party cookies.

Proposal

Determine how we can maintain full feature availability in the Web IDE without requiring someone to accept third-party cookies from their GitLab domain.