Add top bar in standalone gallery and notebook viewer (#76)

* Add top bar in standalone gallery

* Address review comments

src/Explorer/Controls/Header/GalleryHeaderComponent.tsx

@@ -0,0 +1,57 @@
+import * as React from "react";
+import { Stack, Text, Separator, FontIcon, CommandButton, FontWeights } from "office-ui-fabric-react";
+
+export class GalleryHeaderComponent extends React.Component {
+  private static readonly headerText = "Microsoft Azure";
+  private static readonly cosmosdbText = "Cosmos DB";
+  private static readonly galleryText = "Gallery";
+  private static readonly loginText = "Log in";
+  private static readonly loginOnClick = () => (window.location.href = new URL("./", window.location.href).href);
+  private static readonly textStyle: React.CSSProperties = {
+    color: "white"
+  };
+
+  public render(): JSX.Element {
+    return (
+      <Stack
+        tokens={{ childrenGap: 10 }}
+        horizontal
+        styles={{ root: { background: "#0078d4", paddingLeft: 20, paddingRight: 20 } }}
+        verticalAlign="center"
+      >
+        <Stack.Item>
+          <Text
+            style={GalleryHeaderComponent.textStyle}
+            variant="mediumPlus"
+            styles={{ root: { fontWeight: FontWeights.semibold } }}
+          >
+            {GalleryHeaderComponent.headerText}
+          </Text>
+        </Stack.Item>
+        <Stack.Item>
+          <Separator vertical />
+        </Stack.Item>
+        <Stack.Item>
+          <Text style={GalleryHeaderComponent.textStyle}>{GalleryHeaderComponent.cosmosdbText}</Text>
+        </Stack.Item>
+        <Stack.Item>
+          <FontIcon style={GalleryHeaderComponent.textStyle} iconName="ChevronRight" />
+        </Stack.Item>
+        <Stack.Item>
+          <Text style={GalleryHeaderComponent.textStyle}>{GalleryHeaderComponent.galleryText}</Text>
+        </Stack.Item>
+        <Stack.Item grow>
+          <></>
+        </Stack.Item>
+        <Stack.Item>
+          <CommandButton
+            style={GalleryHeaderComponent.textStyle}
+            text={GalleryHeaderComponent.loginText}
+            ariaLabel={GalleryHeaderComponent.loginText}
+            onClick={GalleryHeaderComponent.loginOnClick}
+          />
+        </Stack.Item>
+      </Stack>
+    );
+  }
+}