New domains config source fetching information from GitLab

Description

After we merge !188 (merged) and !189 (merged) we will be able to start building new pages' configuration source.

Previously the only configuration source we had was a hierarchy of groups / directories persisted on the disk and some config.json files written there holding configuration.

We want to phase this configuration source out and move towards microservice-like GitLab <- Pages communication using APIs.

Proposal

We now need to implement a new source that will fetch information about domains from GitLab.

This needs to use caching (do we need etags?), and domain TTLs.

This has been partially done as a PoC in !135 (closed)

/cc @krasio @vshushlin