WIsh: Allow selecting which decks to import

I wanted to export a 25 card deck, but apparently I had installed several other decks using web sources that I'm not actively using, so the extension is very, very slowly importing over 40,000 cards so I can access the 1 deck with 25 cards that I'm interested in.

The issue does not name files, tests, or an entry point. Start by locating the deck import flow and its selection stage; done means allowing a user to choose which installed web-source decks to import instead of importing every deck, including the reported 25-card versus 40,000-card case.

索引モデルが issue の本文から書いたものです。