Empty deck before importing

[yoganlava]

Summary

Empty deck before importing

Closes #255

App changes (app/, etc.):

• Empties deck before importing

Testing

Have you have tested your changes in the following scenarios? Feel free to check off scenarios which don't apply.

• [x] Starting backend services locally with docker compose up succeeds.
• [x] I am able to create a new user and log in locally.
• [x] I am able to complete a practice game locally.
• [x] I am able to complete a purchase of Orbs, etc.

[willroberts]

Thanks!

Feel free to send me your Discord handle so I can give you the contributor role in the OpenDuelyst server.

[yoganlava]

Sure it's Lava#1337