(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/models/DocModel.ts

@@ -36,6 +36,8 @@ import {createValidationRec, ValidationRec} from 'app/client/models/entities/Val
 import {createViewFieldRec, ViewFieldRec} from 'app/client/models/entities/ViewFieldRec';
 import {createViewRec, ViewRec} from 'app/client/models/entities/ViewRec';
 import {createViewSectionRec, ViewSectionRec} from 'app/client/models/entities/ViewSectionRec';
+import {GristObjCode} from 'app/plugin/GristData';
+import {decodeObject} from 'app/plugin/objtypes';
 
 // Re-export all the entity types available. The recommended usage is like this:
 //    import {ColumnRec, ViewFieldRec} from 'app/client/models/DocModel';
@@ -95,6 +97,24 @@ export function refRecord<TRow extends MetaRowModel>(
   return ko.pureComputed(() => tableModel.getRowModel(rowIdObs() || 0, true));
 }
 
+type RefListValue = [GristObjCode.List, ...number[]]|null;
+/**
+ * Returns an observable with a list of records from another table, selected using RefList column.
+ * @param {TableModel} tableModel: The model for the table to return a record from.
+ * @param {ko.observable} rowsIdObs: An observable with a RefList value.
+ */
+export function refListRecords<TRow extends MetaRowModel>(
+  tableModel: MetaTableModel<TRow>, rowsIdObs: ko.Observable<RefListValue>|ko.Computed<RefListValue>
+) {
+  return ko.pureComputed(() => {
+    const ids = decodeObject(rowsIdObs()) as number[]|null;
+    if (!Array.isArray(ids)) {
+      return [];
+    }
+    return ids.map(id => tableModel.getRowModel(id, true));
+  });
+}
+
 // Use an alias for brevity.
 type MTM<RowModel extends MetaRowModel> = MetaTableModel<RowModel>;