Soft delete phabricator_repos when associated services/repos are soft deleted

Created by: flying-robot

Note: still WIP

Although the phabricator_repos table supports soft deletion, we don't use it - no code reads from or modifies the deleted_at timestamp. This leads to confusing bugs[0] for customers, such as "View on Phabricator" rendering in the UI with an incorrect link even after the external service is removed.

TODO

• need to confirm recreation works as expected

[0] https://sourcegraph.atlassian.net/browse/COREAPP-213