Implement Webdav back-ups

This might be a good library to start with: https://pub.dev/packages/webdav_client#upload-file

Special care should be taken to resolve merge conflicts as well as to store the user's password in a secure way (if it's necessary to store it at all).

Rather than syncing the database file directly, it might be worth it to sync just workouts.

Use the "drift" branch to implement this, NOT MASTER.