Implement storage and api for gate survey questions

[jrobbins]

This should resolve #4695. It is the first part of support self-certification of review gates. Basically, each review team can define some survey questions to screen feature owner review requests. If the answers to those questions indicate that the change is not a risk, then the feature owner will be able to click a button to get approval for that gate rather than wait for human review.

In this PR:

• Define a new sub-model of Gate to hold survey question answers.
• Add the survey answers to the converter logic and response openapi models when doing a GET on a Gate.
• Add a SurveyAnswers openapi model to the requests that edit a gate.
• Add logic to the GatesAPI to set either the existing assignee field or the new survey question answers field.
• Add logic to determine when a gate is eligible for self certification based on the Privacy team survey questions.
• Also, change the gate editing request type from POST to PATCH because that is more appropriate for editing, but keep backward compatibility by still handling POST for now.

[DanielRyanSmith]

Time flies!

[DanielRyanSmith]

Optional nit: This file could be named self_certify_helpers.py since it contains no classes and consists only of helper functions

[jrobbins]

Hmm, it would seem odd to have an X_helpers file without having the X file first. I'll stick with the shorter name for now, but keep _helpers in mind.