Implement pipeline reject filter for github

Parts of this were already added, the schema and even a test layout that
used the option. However it was never fully plumbed into the github
model. This change completes that work and adjusts a test to exercise
the reject filter.

Story: 2000774
Task: 4633

Change-Id: I632b71a0ea3d8c6ba0e92f22595d38684705dbe8
3 files changed