Add table names to ActiveRecord scopes to make available after joins

**Run:**

    Offer.joins(issue: :repo).where(repos: {uuid: proj_uuid}).unassigned

**Expected behavior:** only show unassigned issues in specified repository.

**Actual behavior:** error message that column `uuid` is ambiguous.

**Solution:** add the table name `offers` to the scope that defines `.unassigned`

**Note:** This issue might exist in other scopes as well.