[LM-236/LM-268] Add default banner to App template

:koala: :+1:

/cc @zendesk/quokka @maximeprades 

### Description
Adds a default banner image.

### References
 - Jira story: https://zendesk.atlassian.net/browse/LM-236
 - Jira sub-task: https://zendesk.atlassian.net/browse/LM-268

### Risks
  - Developers accidentally submit apps with this default banner image without modifying or removing it which users could take as an implication the app is developed or supported by Zendesk