Add Thunderbird sponsorship banner to home, uploads and download page

server/config.js

@@ -284,6 +284,11 @@ const conf = convict({
     default: '',
     env: 'SEND_DOWNLOAD_NOTICE_HTML'
   },
+  show_thunderbird_sponsor: {
+    format: Boolean,
+    default: false,
+    env: 'SHOW_THUNDERBIRD_SPONSOR'
+  },
   ui_color_primary: {
     format: String,
     default: '#0a84ff',