Fixes render glitch on space selector screen (#151386)

## Summary

When scrollbars are always on, the render of the background image on the
space selector screen was part
<img width="663" alt="Screenshot 2023-02-15 at 6 19 09 PM"
src="https://user-images.githubusercontent.com/103939324/219214315-529a38d8-3cf1-4ce7-8f1e-ca84ffc731bf.png">
ially overlapping the scroll bar in Chrome and Firefox.


This was fixed by overriding the position setting of the
kibanaFullScreenGraphics mixin in .spcSpaceSelector.
<img width="669" alt="Screenshot 2023-02-15 at 6 17 52 PM"
src="https://user-images.githubusercontent.com/103939324/219214099-b317fb53-28aa-4e60-b648-bb98757330c5.png">

Additionally, it was noticed that the telemetry banner, if not
dismissed, had been visually hidden in this screen when it should have
been shown. This is now also resolved.

### Test
- Set scrollbars to always on (MacOS: Settings->Appearance->Show scroll
bars->Always)
- Start ES & Kibana - do NOT dismiss the telemetry banner
- Add multiple spaces
- Navigate to [base URL]/spaces/space_selector
- Adjust window size to cause scrollbar to appear and background
graphics to align with it (reference above screenshot)
- The background graphics should not render over the scroll bar
- The telemetry banner should render in front and at the top of the
window

---------

Co-authored-by: Constance Chen <constance.chen@elastic.co>
Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/spaces/public/space_selector/__snapshots__/space_selector.test.tsx.snap

@@ -6,9 +6,15 @@ exports[`it renders with custom logo 1`] = `
   data-test-subj="kibanaSpaceSelector"
   panelled={true}
 >
+  <EuiPortal>
+    <div
+      className="spcSelectorBackground"
+      role="presentation"
+    />
+  </EuiPortal>
   <_EuiPageSection
-    className="spcSpaceSelector__pageContent"
     color="transparent"
+    paddingSize="xl"
   >
     <EuiText
       size="s"
@@ -63,9 +69,15 @@ exports[`it renders without crashing 1`] = `
   data-test-subj="kibanaSpaceSelector"
   panelled={true}
 >
+  <EuiPortal>
+    <div
+      className="spcSelectorBackground"
+      role="presentation"
+    />
+  </EuiPortal>
   <_EuiPageSection
-    className="spcSpaceSelector__pageContent"
     color="transparent"
+    paddingSize="xl"
   >
     <EuiText
       size="s"

x-pack/plugins/spaces/public/space_selector/space_selector.scss

@@ -1,10 +1,11 @@
 .spcSpaceSelector {
-  @include kibanaFullScreenGraphics;
+  background: transparent;
 }
 
-.spcSpaceSelector__pageContent {
-  position: relative;
-  z-index: 10;
+.spcSelectorBackground {
+  @include kibanaFullScreenGraphics;
+  z-index: -1;
+  pointer-events: none;
 }
 
 // Fix forced focus outline on text that isn't a link to just be an underline

x-pack/plugins/spaces/public/space_selector/space_selector.tsx

@@ -12,6 +12,7 @@ import {
   EuiImage,
   EuiLoadingSpinner,
   EuiPanel,
+  EuiPortal,
   EuiSpacer,
   EuiText,
   EuiTextColor,
@@ -121,7 +122,12 @@ export class SpaceSelector extends Component<Props, State> {
         data-test-subj="kibanaSpaceSelector"
         panelled
       >
-        <KibanaPageTemplate.Section className="spcSpaceSelector__pageContent" color="transparent">
+        {/* Portal the fixed background graphic so it doesn't affect page positioning or overlap on top of global banners */}
+        <EuiPortal>
+          <div className="spcSelectorBackground" role="presentation" />
+        </EuiPortal>
+
+        <KibanaPageTemplate.Section color="transparent" paddingSize="xl">
           <EuiText textAlign="center" size="s">
             <EuiSpacer size="xxl" />
             {this.state.customLogo ? (