(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/ui/GridViewMenus.ts

@@ -37,7 +37,7 @@ export interface IMultiColumnContextMenu {
   // true for some columns, but not all.
   numColumns: number;
   numFrozen: number;
-  disableModify: boolean|'mixed';  // If the columns are read-only.
+  disableModify: boolean|'mixed';  // If the columns are read-only. Mixed for multiple columns where some are read-only.
   isReadonly: boolean;
   isRaw: boolean;
   isFiltered: boolean;            // If this view shows a proper subset of all rows in the table.