feat: hubs

[isTravis]

Hubs are an additive organizational layer on top of Communities. They let an institution or organization (e.g. a university press, a research lab) group multiple PubPub Communities under a single branded umbrella.

What a Hub provides

• A branded landing page (/hub/:slug) that showcases all of its member Communities
• Org-level branding: hero image, logo, accent colors
• A data dashboard with aggregated details across member Communities
• Hub Managers who can administer the hub independently of individual Community permissions

What it doesn't change

• Communities continue to work exactly as they do today. Hubs don't modify or restrict any existing Community functionality
• Membership in a Hub is opt-in, and Communities can opt out of curation