(core) Comments

Summary:
First iteration for comments system for Grist.
- Comments are stored in a generic metatable `_grist_Cells`
- Each comment is connected to a particular cell (hence the generic name of the table)
- Access level works naturally for records stored in this table
-- User can add/read comments for cells he can see
-- User can't update/remove comments that he doesn't own, but he can delete them by removing cells (rows/columns)
-- Anonymous users can't see comments at all.
- Each comment can have replies (but replies can't have more replies)

Comments are hidden by default, they can be enabled by COMMENTS=true env variable.
Some things for follow-up
- Avatars, currently the user's profile image is not shown or retrieved from the server
- Virtual rendering for comments list in creator panel. Currently, there is a limit of 200 comments.

Test Plan: New and existing tests

Reviewers: georgegevoian, paulfitz

Reviewed By: georgegevoian

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3509

app/client/models/features.ts

@@ -0,0 +1,12 @@
+import {getGristConfig} from 'app/common/urlUtils';
+import {get as getBrowserGlobals} from 'app/client/lib/browserGlobals';
+import {localStorageBoolObs} from 'app/client/lib/localStorageObs';
+import {Observable} from 'grainjs';
+
+export function COMMENTS(): Observable<boolean> {
+  const G = getBrowserGlobals('document', 'window');
+  if (!G.window.COMMENTS) {
+    G.window.COMMENTS = localStorageBoolObs('feature-comments', Boolean(getGristConfig().featureComments));
+  }
+  return G.window.COMMENTS;
+}