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
