(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/common/DocActions.ts

@@ -79,7 +79,6 @@ export function isRenameTable(act: DocAction): act is RenameTable { return act[0
 const SCHEMA_ACTIONS = new Set(['AddTable', 'RemoveTable', 'RenameTable', 'AddColumn',
   'RemoveColumn', 'RenameColumn', 'ModifyColumn']);
 
-// Maps each data action to whether it's a bulk action.
 const DATA_ACTIONS = new Set(['AddRecord', 'RemoveRecord', 'UpdateRecord', 'BulkAddRecord',
   'BulkRemoveRecord', 'BulkUpdateRecord', 'ReplaceTableData', 'TableData']);