Add table names to ActiveRecord scopes to make available after joins

[GeorgLink]

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.