When sorting by custom key put untagged archives at the back

[psilabs-dev]

When displaying pages of results as a result of calling the search API with a custom sort key (e.g. "source") in descending order, LRR will present untagged archives first, then followed by tagged archives correctly sorted by "source" in descending order. This is because in sort_results the sort values of untagged archives are automatically "zzzz".

I think that whenever sorting is called on a custom key, untagged archives should always be put to the back, because when a user is sorting by a key they're already expecting archives to have the key. Sorting by "source" in descending order has relevance in the context of many sites whose ID generation is increment-based.

A branch on this is in progress. (I don't know how long this will take)

[Difegue]

Please make sure you repull if you're working on this due to the changes to sorting in particular caused by #1296 .
(Although the sorting itself retains the same logic with the fallback to zzzz so it shouldn't be too hard to reconcile)