Delete assets in restricted bucket

As a user I want to be able to delete assets in restricted bucket.

AC

• Existing /media_items DELETE REST endpoint is modified to accept assets in restricted bucket. Same as with creation, a query parameter restricted is added
• Deletion is allowed for users with roles Admin and Project Lead
• corresponding thumbnails are automatically deleted
• Direct deletion of thumbnails is not allowed
• Tests

Review

• Existing /media_items DELETE REST endpoint is modified to accept assets in restricted bucket. Same as with creation, a query parameter restricted is added
• Deletion is allowed for users with roles Admin and Project Lead
• corresponding thumbnails are automatically deleted
• Direct deletion of thumbnails is not allowed
• Tests