[WebIDE] Allow GitLab self-managed admins to provide custom Web IDE extensions host domain

Problem to solve

GitLab Self-Managed instances that run on air-gapped environments can't connect to the .gitlab-static.net extension host domain therefore many capabilities in the Web IDE don't work. Instance administrators can't provide a custom extension host domain either therefore they end up with a broken Web IDE experience.

Solution proposal

Create an application-level setting that allows instance administrators to provide a custom Web IDE extension host domain.

Acceptance criteria

• Add a section "Web IDE" to the GitLab instance admin settings page.
• The "Web IDE" section has a form that allows editing the extension host domain.
• The extension host domain form field only allows entering valid domain names.
• The extension host domain form field default value is .gitlab-static.net.
• The extension host domain form field allows resetting to the default value.

Implementation ideas