New default background (#8597)

* feat(bgs): new default background

* feat(bgs): backfill migration

* fix(migration): extraneous imports, bad paths

* fix(bgs): address comments

* fix(test): assert equality

(cherry picked from commit 03088f1d9f)

website/common/script/content/appearance/backgrounds.js

@@ -480,6 +480,11 @@ let backgrounds = {
     },
   },
   incentiveBackgrounds: {
+    violet: {
+      text: t('backgroundVioletText'),
+      notes: t('backgroundVioletNotes'),
+      currency: 'loginIncentive',
+    },
     blue: {
       text: t('backgroundBlueText'),
       notes: t('backgroundBlueNotes'),