Load domain data from API instead of traversing filesystem

We've seen a number of bugs where stale values of config.json are stored or stale directories mess up the domains loading. I think it might make sense to switch GitLab Pages to load the data from the API. We are doing this to support access control in !94 (merged).

• CE: Add API endpoint to serve project pages config.json
• Pages: Fetch and cache config when it does not exist