X-ray preview banner (#6051)

* add preview banner

* add icon / survey link

frontend/src/metabase/xray/components/PreviewBanner.jsx

+import React from 'react'
+import Icon from 'metabase/components/Icon'
+
+const SURVEY_LINK = 'https://docs.google.com/forms/d/e/1FAIpQLSc92WzF76ViiT8l4646lvFSWejNUhh4lhCSMXdZECILVwJG2A/viewform?usp=sf_link'
+
+const PreviewBanner = () =>
+    <div className="full py2 flex align-center justify-center full md-py3 text-centered text-slate text-paragraph bg-white border-bottom">
+        <Icon name='beaker' size={28} className="mr1 text-brand" style={{ marginTop: -5 }} />
+        <span>Welcome to the x-ray preview! We'd love <a className="link"  href={SURVEY_LINK} target="_blank">your feedback.</a></span>
+    </div>
+
+export default PreviewBanner

frontend/src/metabase/xray/components/XRayLayout.jsx

 import React from 'react'
 import { withBackground } from 'metabase/hoc/Background'
+import PreviewBanner from 'metabase/xray/components/PreviewBanner'
 
 // A small wrapper to get consistent page structure
 export const XRayPageWrapper = withBackground('bg-slate-extra-light')(({ children }) =>
-    <div className="XRayPageWrapper wrapper pb4 full-height">
-        { children }
+    <div className="full-height full">
+        <PreviewBanner />
+        <div className="XRayPageWrapper wrapper pb4 full-height">
+            { children }
+        </div>
     </div>
 )