Allow arbitrary sorting of price alerts

Storing a `position` field for price alerts, which could be `NULL`, we should support sorting price alerts by `position`. `NULL` values should be sorted to the button, and the alerts should otherwise appear in the insertion order they appear in currently. (Order by position, nulls last, then order by `id`.)

Implementation detail: Updating the position of elements will require updating all other alerts for a user to shift the values and database transactions.