Initial Fabric support (#1607)

* Add Platform.Fabric to run in context of Fabric

* Use separate StyleConstants

We want to have more flexibility with Styles at runtime
but Constants depend on ConfigContext and therefore
get loaded very early at startup.

* Add Fabric specific styles and Fluent theme

documentDBFabric.less contains all styles for Fabric.
We use React.lazy to import them conditionally at
runtime preventing webpack from preprocessing
them into main.css.

* Restyle CommandBar for Fabric
with more roundness and native colors.

* Disable Notebooks when running in Fabric

* Disable Synapse and Scripts commands for Fabric

* Fix code formatting issues

* Fetch encrypted token from sessionStorage for fabric platform

* Fix Tabs style

* Dark refresh icons for Fabric

* Use new ResourceTree2 for Fabric

* Fluent tree should have a fixed width
otherwise the action buttons jump around on hover.

* Disable remaining Script actions in Fabric

* Revert accidentally committed change
which broke a test

* Fix cross-origin error second try

* Adjust @FabrixBoxMargin css

* Hide Database Scale node on Fabric

* Remove all Collection child nodes on Fabric

* Add a comment about why we need FabricPlatform.tsx

* Fix equality checks

* Fix more Colors for Fabric

* Switch resource tree to "medium" size

* Fix formatting again

* Fix formatting again

* Disable no-var-requires error on some intended var import.

* Increase memory limit for build

* Use standard Segoe UI font for Fabric

* Improve Tabs design for Fabric

* Fix active Tab style bug in Portal
introduced with 39a7765aef

---------

Co-authored-by: Laurent Nguyen <laurent.nguyen@microsoft.com>

src/Explorer/Controls/TreeComponent/TreeComponent.tsx

@@ -18,6 +18,7 @@ import LoadingIndicator_3Squares from "../../../../images/LoadingIndicator_3Squa
 import TriangleDownIcon from "../../../../images/Triangle-down.svg";
 import TriangleRightIcon from "../../../../images/Triangle-right.svg";
 import * as Constants from "../../../Common/Constants";
+import { StyleConstants } from "../../../Common/StyleConstants";
 import { Action, ActionModifiers } from "../../../Shared/Telemetry/TelemetryConstants";
 import * as TelemetryProcessor from "../../../Shared/Telemetry/TelemetryProcessor";
 
@@ -237,7 +238,7 @@ export class TreeNodeComponent extends React.Component<TreeNodeComponentProps, T
   private renderContextMenuButton(node: TreeNode): JSX.Element {
     const menuItemLabel = "More";
     const buttonStyles: Partial<IButtonStyles> = {
-      rootFocused: { outline: `1px dashed ${Constants.StyleConstants.FocusColor}` },
+      rootFocused: { outline: `1px dashed ${StyleConstants.FocusColor}` },
     };
 
     return (