chore: Contact import improvements (#7787)

- Ensure existing contact information is updated on data import
- Refactor the existing job to make it more readable
- Fixes issues with import files in the wrong encoding
fixes: #7307

app/javascript/dashboard/i18n/locale/en/contact.json

@@ -71,7 +71,7 @@
       "SUBMIT": "Import",
       "CANCEL": "Cancel"
     },
-    "SUCCESS_MESSAGE": "Contacts saved successfully",
+    "SUCCESS_MESSAGE": "You will be notified via email when the import is complete.",
     "ERROR_MESSAGE": "There was an error, please try again"
   },
   "EXPORT_CONTACTS": {