Add referrer assigned, received, and paid-job notification types

[Amtron]

Summary

• Add REFERRER_ASSIGNED (105), REFERRER_JOB_PAID (106), and REFERRER_JOB_RECEIVED (107) to the Python and PHP notification type constants.
• Put assigned and received on the domain-avatar list and paid-job on the payment avatar list so clients can render them.

Test plan

• Confirm REFERRER_ASSIGNED == 105, REFERRER_JOB_PAID == 106, and REFERRER_JOB_RECEIVED == 107
• Confirm all three keys exist in NOTIFICATION_TYPE_STRINGS

[Unitix]

LGTM