PlayerPurchases table desync with save import

[SapiensAnatis]

Describe the bug
The PlayerPurchases table is not cleared when importing a save.

This can lead to desync. For example, suppose you were to be on an endgame savefile with all dragon storage upgrades purchased, and then imported a fresh savefile to start over. In this situation, you would have the minimum amount of dragon storage but be unable to purchase any more as it would show that you had already purchased all the available upgrades.

Expected behavior
The dragon storage purchase would reset in this scenario.

Additional context
If we just clear the table on save import, this would lead to exploits where you can re-import the same savefile and purchase more storage. We probably need to add extra data to the savefile format.

[SapiensAnatis]

This should be blocked by #1092