Warn expiring users

[missytake]

closes #21

the idea is to warn users a certain timespan before their accounts run out:

• if their token is 1year or longer, a month, a week, and a day before expiration date
• if their token is 1month or longer, a week and a day before expiration date
• if their token is 1week or longer, a day before expiration date
• if it's shorter than that, after 3/4 of the expiration notice, they get told how much they have left either in minutes or hours.

right now, the expiration warnings are triggered by the prune loop; this is a bit unfortunate as it means that we have to initialize another deltachat.account object for the mailadm bot, and can't reuse the bot object. Maybe that's fine, it just complicates the handling a bit.

I tried it out manually, works so far. There are 2 tests, one of them could be more minimal I guess.

Does the database migration need a test?

[missytake]

That's how it looks so far - it would be reeeeally nice to have this in a legit "provider chat" in the future <3

There is https://github.com/deltachat/mailadm/pull/57 to make this a bit nicer already :)

[missytake]

We called and decided that we will not support the use case of running mailadm without a bot; the prune thread will only be started as soon as we have a working bot account object.

[missytake]

This needs to be done again, but it only makes sense after we decided on #78.