Speed up Pages initialization time by using configuration API

Description

Once the Pages configuration API is in place it will be necessary to update gitlab-pages to load its configuration from the API instead of the filesystem. This should be done in a way that will allow support for both implementations (API and config file) for a period of time. Shipping two versions of the binary with Omnibus has been considered as an approach.

This will bring the time of pages configuration update from ~30 minutes to almost instant in gitlab.com, including:

1. time for the project to be visible for the first time
2. adding new domains
3. adding SSL certificates

Proposal

This work has already started in this MR, finishing up that work will complete this issue.

Links / references

Related Epic: &1316 (closed)