Expose repository-level health information via a repository dashboard

Customers do not currently have a way to surface fine-grained details around their repository. All they have access to is the information how big that repository is. This means that those customers cannot easilyi figure out performance-related information around repositories on their own without either involving support or executing Git commands in the on-server repository itself.

We should provide a UI to surface repository-level information:

• What is the repository's total size?
• Which format does the repository use?
• Does the repository have supporting metadata like commit graphs, bitmaps and multi-pack indices? How old are they?
• How large are the objects?
• How many objects are there?
• Which types do these objects have?
• How many refs are there?

Gitaly already exposes a lot of this information via the RepositoryInfo() RPC. We should expose that information to make repositories more self-serviceable and improve the handoff between customers and GitLab.