(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

app/client/lib/koUtil.js

@@ -63,6 +63,14 @@ ko.subscribable.fn.onlyNotifyUnequal = function() {
   return this;
 };
 
+/**
+ * Notifies only about distinct defined values. If the first value is undefined it will still be
+ * returned.
+ */
+ko.subscribable.fn.previousOnUndefined = function() {
+  this.equalityComparer = function(a, b) { return a === b || b === undefined; };
+  return this;
+};
 
 let _handlerFunc = (err) => {};
 let _origKoComputed = ko.computed;
@@ -73,8 +81,8 @@ let _origKoComputed = ko.computed;
  * evaluates successfully to its previous value (or _handlerFunc may rethrow the error).
  */
 function _wrapComputedRead(readFunc) {
+  let lastValue;
   return function() {
-    let lastValue;
     try {
       return (lastValue = readFunc.call(this));
     } catch (err) {