(core) Add 'Help us improve' form to QuickSetup

Summary:
Adds a "Help us improve" form the quick setup flow, allowing self-hosted users to submit some optional details and opt-in to product and security updates.

This adds:
- The form itself to Grist core's UI
- An endpoint to our SaaS code that receives the data, and forwards it to a Grist doc and SendGrid.

Test Plan: Manual testing against a local doc, unit tests covering the new endpoint, browser tests covering the happy path.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: georgegevoian

Differential Revision: https://phab.getgrist.com/D5002