Embed auth token into HTML

Summary:
Auth token used be injected in the manifest file. Instead, have the server injected into the main HTML page.

The main driver to this change are:
- Simplify
- There are instances in which for some reason reading/writing the token from the manifest fails. This will address that problem.

Reviewed By: lblasa

Differential Revision: D51160521

fbshipit-source-id: 4626fd8f56bc8b61182a53a5d9cf5acad1e723bc

desktop/flipper-ui-browser/src/global.tsx

@@ -22,6 +22,7 @@ declare global {
     FLIPPER_APP_VERSION: string;
     FLIPPER_SESSION_ID: string;
     FLIPPER_UNIXNAME: string;
+    FLIPPER_AUTH_TOKEN: string;
 
     flipperShowMessage?(message: string): void;
     flipperHideMessage?(): void;