Following on from #1146 this PR removes all unused exports from
`loot-core/`

- Enforce that imports from the same package are merged into a single
import
- In `loot-core`, require that imports of other `loot-core` files use a
relative import (like the vast majority of such imports) rather than
specifiers starting with `loot-core/` (probably a result of moving files
out of other packages into `loot-core`)

Another batch of `loot-core` migrated.

* Remove invalid ref={} prop

* Fix <rect> height being less than 0

* Improve history.push override

* Filters: add missing key prop

* DateSelect: don’t pass `null` to an input

* Filters: add missing key prop

* EditRule: fix key props

* ScheduleDetails: Fix `for` prop

* usePayees: default `payees` to empty array instead of null (fixes crash)

* safeNumber: better error if `value` is not a number

* ScheduleDescription: fix description for “amount is between”

* ScheduleDetails: add missing `key` prop

* BudgetTitlebar: use ButtonWithLoading

* style: enforce ordering of imports

* style: sort imports in loot-core

* style: sort imports in desktop-client

* style: sort imports in loot-design

* style: manual fixes