(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/server/lib/RowAccess.ts

@@ -76,3 +76,15 @@ export function getRowIdsFromDocAction(docActions: RemoveRecord | BulkRemoveReco
   const ids = docActions[2];
   return (typeof ids === 'number') ? [ids] : ids;
 }
+
+/**
+ * Tiny helper to get the row ids mentioned in a record-related DocAction as a list
+ * (even if the action is not a bulk action). When the action touches the whole row,
+ * it returns ["*"].
+ */
+export function getColIdsFromDocAction(docActions: RemoveRecord | BulkRemoveRecord | AddRecord |
+  BulkAddRecord | UpdateRecord | BulkUpdateRecord | ReplaceTableData |
+  TableDataAction) {
+  if (docActions[3]) { return Object.keys(docActions[3]); }
+  return ['*'];
+}