Summary:
Adds a new Support Grist page (accessible only in grist-core), containing
options to opt in to telemetry and sponsor Grist Labs on GitHub.
A nudge is also shown in the doc menu, which can be collapsed or permanently
dismissed.
Test Plan: Browser and server tests.
Reviewers: paulfitz, dsagal
Reviewed By: paulfitz
Subscribers: jarek, dsagal
Differential Revision: https://phab.getgrist.com/D3926
