Instead of having the foreign key `repository_id` in the `projects`
table, have a FK in the `project_repositories` table. This will:

- Reduce the size of `projects` table, which is huge already.
- Make the foreign key constraint `on_delete: :cascade` useful,
  because the `project_repositories` row will be deleted when
  `projects` row is deleted.
- Since the `project_repositories` row can only be created **after**
  the `projects` row (because hashed storage needs a `projects.id`),
  it does not require updating the `projects` row after creating the
  `project_repositories`.
- Make it easier in the future to have a N-to-M relation.