fix: server to randomize image and logo filenames before generating presigned url

[tshuli]

Problem

Fixes #128

Solution

• Backend prefixes random string to image and logo filenames before returning presigned url, hence ensuring filename in bucket is unpredictable to client when /presign endpoint is first called
• Backward compatibility is ensured by setting isNewClient flag for the frontend, which will be removed at a later date

Tests

• Create storage mode form. Test that logo and image upload works and can be displayed in public form.
• (safety check, should not be affected by this PR) Add attachment field. Test that file upload works and can be retrieved from storage mode
• Repeat with old client
  (this can be done by rolling back to commit d979d0c1)