Allow arbitrary sorting of price alerts

[w0rp]

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.