(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/docmodel.py

@@ -147,6 +147,21 @@ class MetaTableExtras(object):
       table.docmodel.setAutoRemove(rec, not rec.colRef)
 
 
+  class _grist_Cells(object):
+    def setAutoRemove(rec, table):
+      if rec.type == 1: # Cell info of type 1 == Comments
+        # Remove if discussion is removed.
+        noParent = not rec.root and not rec.parentId
+        if rec.tableRef and rec.rowId:
+          tableRef = table.docmodel.get_table(rec.tableRef.tableId)
+          row = tableRef.lookupOne(id=rec.rowId)
+        else:
+          row = False
+        # Remove if row is removed, column is removed, table is removed or all comments are removed.
+        no_cell = not rec.colRef or not rec.tableRef or not row
+        table.docmodel.setAutoRemove(rec, noParent or no_cell)
+
+
 def enhance_model(model_class):
   """
   Given a metadata model class, add all members (formula methods) to it from the same-named inner
@@ -198,6 +213,7 @@ class DocModel(object):
     self.aclResources            = self._prep_table("_grist_ACLResources")
     self.aclRules                = self._prep_table("_grist_ACLRules")
     self.filters                 = self._prep_table("_grist_Filters")
+    self.cells                   = self._prep_table("_grist_Cells")
 
   def _prep_table(self, name):
     """