Export Badge views: improve queries performance
Export Badge views: improve queries performance

ExportBadgesView generated one additional query for each award
displayed, due to not fetching all related persons by default (we're
using a complex intermediate-through many-to-many relationship here).

ExportBadgesByPersonView rendered a strange,
`django.db.utils.OperationalError: too many SQL variables` error.  It
was probably caused by too many IDs listed in the queries.
The error was fixed (probably until we grow our number of people with
badge significantly) by only returning people who have some badge.

vahtras/amy,vahtras/amy,vahtras/amy,swcarpentry/amy,swcarpentry/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy,swcarpentry/amy