Add repositories to Diviner

Summary: Fixes T8352. Associate Diviner books and atoms with a repository. This relationship is not really surfaced anywhere in the UI but provides metadata that contextualises search results. Depends on D13091.

Test Plan: Ran diviner generate --repository ARC and then went to /diviner/book/arcanist/.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7703, T8352

Differential Revision: https://secure.phabricator.com/D13070