(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

sandbox/grist/schema.py

@@ -15,7 +15,7 @@ import six
 
 import actions
 
-SCHEMA_VERSION = 32
+SCHEMA_VERSION = 33
 
 def make_column(col_id, col_type, formula='', isFormula=False):
   return {
@@ -317,6 +317,21 @@ def schema_create_actions():
       # Ex2: { excluded: ['apple', 'orange'] }
       make_column("filter",         "Text")
     ]),
+
+    # Additional metadata for cells
+    actions.AddTable('_grist_Cells', [
+      make_column("tableRef",       "Ref:_grist_Tables"),
+      make_column("colRef",         "Ref:_grist_Tables_column"),
+      make_column("rowId",          "Int"),
+      # Cell metadata is stored as in hierarchical structure.
+      make_column("root",           "Bool"),
+      make_column("parentId",       "Ref:_grist_Cells"),
+      # Type of information, currently we have only one type Comments (with value 1).
+      make_column("type",           "Int"),
+      # JSON representation of the metadata.
+      make_column("content",        "Text"),
+      make_column("userRef",        "Text"),
+    ]),
   ]