[UX Polish] Improve how users opt-in for extension marketplace

Problem to solve

Discuss ways to improve the MVC solution in https://gitlab.com/gitlab-org/gitlab-web-ide/-/issues/289 when enabling the new extension marketplace. The current solution has high UX friction by introducing legalese and using a modal.

Proposed solutions

1. Use streamlined inline text with a checkbox to get opt-in instead of a modal.
2. Detect user's location and only disable the marketplace when it's relevant to their jurisdiction so that we don't degrade the UX for everyone.