Make UserMediaUpload for every existing Media

[mattia-danese]

Summary / Highlights

Django admin page Media function that iterates over all existing Media in the database (regardless of which instances are selected in the Django admin page) and creates a UserMediaUpload if one does not already point to that Media.

Details (Give details about what this PR accomplishes, include any screenshots etc)

Whether a UserMediaUpload exists already for a given Media or not, the UserMediaUpload will point to the Media and store any relevant information (i.e. if the Media is used in an action).

Testing Steps (Provide details on how your changes can be tested)

Note specific Media with known use cases (in action, or vendor, etc.) and ensure the created UserMediaUpload is correct.
Ensure a UserMediaUpload is not created for a Media that already has a UserMediaUpload pointing to it.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've tested my changes manually.
• I've added unit tests to my PR.

Transparency (Project board)

• I've given my PR a meaningful title.
• I linked this PR to the relevant issue.
• I moved the linked issue from the "Sprint backlog" to "In progress" when I started this.
• I moved the linked issue to "QA Verification" now that it is ready to merge.