(core) Add a Notifications & Automations setup nudge with admin requests

Summary:
Grist features like change & comment notifications, automations, invite
emails, and the AI assistant only work once an install has the plumbing
behind them: an email backend, a Redis queue, an AI provider, the full Grist
edition. When a prerequisite is missing the feature simply isn't there, in some
cases, and the user has no easy way to discover what's missing or how to get it.

This fills the Notifications slot in Document Settings (where it would
otherwise be silent) with a per-feature setup section: a pip meter and step
count per feature, expandable to a description and a labelled checklist, telling
the viewer what's ready, what isn't, and what each unfinished feature is waiting
on. A regular user can press "Ask the admin" to ask whoever runs the install to
enable a step; admins see those requests collected in a new admin-panel item.

The "Ask the admin" feature is implemented on the cheap, using a weak database
structure that should be fine for the purpose but also would be very fair to push back on.

Another possibly weak spot: this nudge (I call it the "herald" for some reason)
disappears if notifications are configured, even if other material is not configured.
It could alternatively be always present, or present as long as something is
unconfigured. But also, notifications were the key feature that a user couldn't
learn about in the UI in some way.

Test Plan: adds tests and storybook

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D4988