Convert regular slots to manual slots as soon as someone joins an empty slot

What does this MR do?

• Please review commit-by-commit!

When adjusting the pickup schedule in a store, right now, the old recurring slots vanish from the store view. That's bad, because they still exist (and show up on the dashboard and profile for users!), and it causes lots of confusion.

Instead of a huge refactoring, use the following "convert to onetime pickup" hack:

• when somebody signs into any slot, the associated pickup is converted to a onetime pickup
• this pickup stays "manual" even if people leave slots again, but can easily be cleaned up

So now, when adjusting a store's pickup schedule, people who signed into slots that no longer will be generated will still be visible in the store. They can either be signed out and notified, or treat this as "reservation claim" for the new slot if it was just a minor change in time.

How confident are you it won't break things if deployed?

99% give or take. The hack is ugly design-wise but I cleaned up some code to make up for it.

Links to related issues

• Closes #860 (closed)

Related tickets

• https://foodsharing.freshdesk.com/a/tickets/1019

How to test

See above ;-)

Checklist

• adjusted test cases accordingly
• no unrelated changes
• asked someone for a code review
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Release notes text

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