Only expire unused accounts

[missytake]

This PR introduces soft expiry according to https://github.com/deltachat/mailadm/issues/21#issuecomment-1453556526:

• It only expires accounts which have not been used in the last TTL/25% time.
• Unless the accounts were supposed to live less than a month.
• This translates to a minimum inactivity of a week, before an account is deleted.

There is a test which specifies in the test parameters when we expect accounts to expire and when we expect them to not expire. This makes it easy to change the policy in the future. Any further suggestions on test cases?

This PR does not introduce a limit to soft expiry - if an account was supposed to live for at least a month, and someone is accessing it at least once a week, we still keep it around. But: both the method and the test are written in a way that we can easily change this behavior in the future.

[missytake]

At some point we should add some documentation on what options the tokens can have, but that's no release blocker I guess.

[missytake]

(CI is broken because dc.develcow.de is broken, but it runs successfully with testrun.org)