UI: Initial render whitespace bug (#11159)

## Addresses #11088 
Updated solution:
https://www.loom.com/share/e3ca30bebde54df5b8cbd8cac6d26780
- First refresh displays the bug: a white bar at the top of the page as
the login page loads.
- Second refresh displays the login page with the bug gone
- Navigate at the end to an error page to confirm it is unaffected,
since the initial solution affected the background color of those pages

## Checklist for submitter
- [x] Changes file added for user-visible changes in `changes/` 
- [x] Manual QA for all new/changed functionality

---------

Co-authored-by: Jacob Shandling <jacob@fleetdm.com>

changes/11088-whitespace-at-login

@@ -0,0 +1 @@
+- Fix a bug where a white bar appears at the top of the login page before the app renders.

frontend/pages/LoginPage/LoginPage.tsx

@@ -49,6 +49,8 @@ const statusMessages: IStatusMessages = {
     "There was an error with single sign-on. Please contact your Fleet administrator.",
 };
 
+const baseClass = "login-page";
+
 const LoginPage = ({ router, location }: ILoginPageProps) => {
   const {
     availableTeams,
@@ -193,7 +195,7 @@ const LoginPage = ({ router, location }: ILoginPageProps) => {
   }, [redirectLocation]);
 
   if (isLoadingSSOSettings) {
-    return <Spinner />;
+    return <Spinner className={`${baseClass}__loading-spinner`} />;
   }
 
   return (

frontend/pages/LoginPage/_styles.scss

@@ -1,3 +1,11 @@
+.login-page {
+  &__loading-spinner.centered {
+    margin: 0;
+    position: absolute;
+    left: 50vw;
+    top: 40vh;
+  }
+}
 .login-form-animation-leave {
   transform: scale(1);
   opacity: 1;

frontend/requestCSRdata.ts

@@ -1,4 +0,0 @@
-export interface IRequestCSRData {
-  email: string;
-  org_name: string;
-}