Allow user to upload multiple pictures at once

[johncmanuel]

Rather than the user uploading one picture at a time, allow the user to upload multiple pictures at once (up to a defined limit).

[johncmanuel]

Note that since this app is currently deployed on Vercel, it is mandatory to use signed URLs to upload files. This is already implemented in the User page, but it needs to be modified to fit multiple files.

References:

1. Google Cloud Storage usage in a Next.js app
2. Vercel's guide on uploading/storing files