(core) Conditional formatting rules

Summary:
Adding conditional formatting rules feature.

Each column can have multiple styling rules which are applied in order
when evaluated to a truthy value.

- The creator panel has a new section: Cell Style
- New user action AddEmptyRule for adding an empty rule
- New columns in _grist_Table_columns and fields

A new color picker will be introduced in a follow-up diff (as it is also
used in choice/choice list/filters).

Design document:
https://grist.quip.com/FVzfAgoO5xOF/Conditional-Formatting-Implementation-Design

Test Plan: new tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D3282

sandbox/grist/migrations.py

@@ -898,3 +898,11 @@ def migration26(tdset):
     new_view_section_id += 1
 
   return tdset.apply_doc_actions(doc_actions)
+
+
+@migration(schema_version=27)
+def migration27(tdset):
+  return tdset.apply_doc_actions([
+    add_column('_grist_Tables_column', 'rules', 'RefList:_grist_Tables_column'),
+    add_column('_grist_Views_section_field', 'rules', 'RefList:_grist_Tables_column'),
+  ])