starter: populate user-id instead of username

[peterpeterparker]

The assets root folder has been moved from username to user_id.

Therefore we have to reflect this change in the starter kit and on the api to populate the og:image differently

1. currently the tag is {{DECKDECKGO_USERNAME}} should we change it in the starter or keep it as it is?

2. in the API, populate this field with the user_id instead of username