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/Tabs/Tabs.tsx

@@ -138,12 +138,7 @@ function TabNav({ tab, active, tabKind }: { tab?: Tab; active: boolean; tabKind?
               />
             )}
           </span>
-          <span
-            className="tabNavText"
-            style={active ? { fontWeight: "bolder", borderBottom: "2px solid rgba(0,120,212,1)" } : {}}
-          >
-            {useObservable(tab?.tabTitle || getReactTabTitle())}
-          </span>
+          <span className="tabNavText">{useObservable(tab?.tabTitle || getReactTabTitle())}</span>
           {tabKind !== ReactTabKind.Home && (
             <span className="tabIconSection">
               <CloseButton tab={tab} active={active} hovering={hovering} tabKind={tabKind} />