(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/components/Importer.ts

@@ -21,7 +21,7 @@ import {icon} from 'app/client/ui2018/icons';
 import {IOptionFull, linkSelect, menu, menuDivider, menuItem, multiSelect} from 'app/client/ui2018/menus';
 import {cssModalButtons, cssModalTitle} from 'app/client/ui2018/modals';
 import {loadingSpinner} from 'app/client/ui2018/loaders';
-import {openFormulaEditor} from 'app/client/widgets/FieldEditor';
+import {openFormulaEditor} from 'app/client/widgets/FormulaEditor';
 import {DataSourceTransformed, DestId, ImportResult, ImportTableResult, MergeOptions,
         MergeOptionsMap, MergeStrategy, NEW_TABLE, SKIP_TABLE,
         TransformColumn, TransformRule, TransformRuleMap} from 'app/common/ActiveDocAPI';

app/client/components/viewCommon.css

@@ -75,7 +75,7 @@
   color: var(--grist-actual-cell-color, black);
 }
 
-.field_clip.invalid {
+.field_clip.invalid, .field_clip.field-error-from-style {
   background-color: #ffb6c1;
   color: black;
 }