nmbug-status: add support for specifying sort order for each view

nmbug-status: add support for specifying sort order for each view

Let each view have a "sort" key, typically used with values
"oldest-first" or "newest-first" (although all values in Query.SORT
are accepted), and sort the results accordingly. Oldest first remains
the default.

The dynamic approach of mapping sort values is as suggested by
W. Trevor King <wking@tremily.us>.