(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/schema.py

@@ -15,7 +15,7 @@ import six
 
 import actions
 
-SCHEMA_VERSION = 26
+SCHEMA_VERSION = 27
 
 def make_column(col_id, col_type, formula='', isFormula=False):
   return {
@@ -83,6 +83,8 @@ def schema_create_actions():
       # E.g. Foo.person may have a visibleCol pointing to People.Name, with the displayCol
       # pointing to Foo._gristHelper_DisplayX column with the formula "$person.Name".
       make_column("visibleCol",       "Ref:_grist_Tables_column"),
+      # Points to formula columns that hold conditional formatting rules.
+      make_column("rules",       "RefList:_grist_Tables_column"),
 
       # Instructions when to recalculate the formula on a column with isFormula=False (previously
       # known as a "default formula"). Values are RecalcWhen constants defined below.
@@ -206,6 +208,8 @@ def schema_create_actions():
       make_column("visibleCol",   "Ref:_grist_Tables_column"),
       # DEPRECATED: replaced with _grist_Filters in version 25. Do not remove or reuse.
       make_column("filter",       "Text"),
+      # Points to formula columns that hold conditional formatting rules for this field.
+      make_column("rules",       "RefList:_grist_Tables_column"),
     ]),
 
     # The code for all of the validation rules available to a Grist document