Add PDF exports for review decks for sharing

### What is the problem

Currently deck is only for you. You cannot share it with anyone, and even if you could, you'll need the other person you have exact set of changes and should have the same deck version. We need a sharable version of decks that can be circulated around.

### What you had in mind

An option a shortcut, that can,
1. Ask me which groups that I wish to export
2. Order the groups properly and give me an overview of how the PDF is going to look like.
3. Export the PDF and save it in my storage.

The issue names no files, tests, or entry points. Start by locating the existing deck, group-selection, ordering, preview, and storage entry points, then determine how PDF generation fits them. Done means users can select groups, review their order and appearance, export a PDF, and save it to storage.

Written by the indexing model from the issue text.