Implement partially-complete VS Code behind a feature flag

Problem to solve

Getting the right people access to the early builds of the new Web IDE would be a lot easier if it was in production.

Proposal

1. Put the new gitlab-web-ide behind a feature flag
2. Map the Web IDE buttons to open the new Web IDE
3. Introduce a simple user preference that switches the behavior of the edit button to use the old (editable) Web IDE and the new (incomplete) Web IDE

Details

The VS Code Web IDE will be incomplete. Most likely you won't be able to make commits. You may not be able to edit a file at all. Whatever state we can get it in where we can at least open the Web IDE in-product, let's get that out as soon as possible. We can iterate on the features faster from there.