Bugfixes before release 2.3.0

[akochari]

Description

This PR fixes two things before release 2.3.0:

• added missing migrations file
• added placeholder for the image field in the app creation/edit form for custom apps

Checklist

If you're unsure about any of the items below, don't hesitate to ask. We're here to help!
This is simply a reminder of what we are going to look for before merging your code.

• This pull request is against develop branch (not applicable for hotfixes)
• I have included a link to the issue on GitHub or JIRA (if any)
• I have included migration files (if there are changes to the model classes)
• I have included, reviewed and executed tests (unit and end2end) to complement my changes
• I have updated the related documentation (if necessary)
• I have added a reviewer for this pull request
• I have added myself as an author for this pull request
• In the case I have modified settings.py, then I have also updated the studio-settings-configmap.yaml file in serve-charts