Discussion categories should be sorted chronologically

By default categories should be sorted chronologically but
it is not showing result becuase of the mongo query and
use of default dict which effect the ordering. Changing
the mongo query and default dict to ordereddict fixed
the problem.

TNL-870
do we have to do this sort in the database?

also, if we must sort in the db, does it make sense to have the chron sort be additive to `SORT_REVISION_FAVOR_DRAFT` ?  I don't know offhand if that sort is doing something meaningful but I'm worried about unexpected side effects.