Declare open groups as authoritative over content

## Problem:
Currently, there is no way for a publisher or website owner to declare an open group as the group that Hypothesis users should annotate in over their site (authoritative over their content).

## User Story:
* As a website owner or a publisher, I want to mark open groups that either I created, or that others created, as authoritative groups over my content, so that I can determine where annotations show up on my page, thus improving the relevance and quality of discussion around articles in order to provide greater value to the end user.
* As a Hypothesis user, I want to see groups that are relevant to the content I'm reading, that have been chosen by the publisher/website owner as authoritative, as it will increase the likelihood that I'll consume the content and/or participate in the discussion.

## Acceptance Criteria:
- [ ] Publishers/website owners can identify a group as authoritative over their domain,
- [ ] Publishers/website owners can identify a group as authoritative over specific parts of their website, instead of over their entire domain.
- [ ] Publishers/website owner must demonstrate that they have control over their page - we don't want anyone to be able to declare a group as authoritative.
- [ ] Group should be configured in such a way that it cannot be declared authoritative anywhere else on the web.

No files, tests, or entry points are identified, so begin by mapping the group-authority and website-control flows in the project. The work is done when publishers can verify control, scope an open group to a domain or site section, and prevent that group from being declared authoritative elsewhere.

Written by the indexing model from the issue text.