Querki needs a privacy statement

Summary: Doesn't have to be perfect, but we need something.

Eric pointed out (email, Oct 11 '20) that GDPR compliance is becoming increasingly important. At the very least, we need a statement that includes a discussion of what information can be deleted. This should restate that data entered into someone else's Space is considered their property, and will not be deleted if you remove yourself from Querki, but you can delete your account and your Spaces.

(We also need to make it possible to delete your account.