Refactor post-signup user logic
Add referrer tracking
Apply a give $20 discount when user orders a first job
Remove referrer_id from session after user signs up
Rename referral attribute
Add User.referrer assoc, add invitations assocs and logic
Refactor give 20 discount system, award get 20 discount when job is paid
Update get20/give20 discount system
Update user receipt
Show half-stars in mechanic rating meter
Fix some specs
Include service parts in job breakdown