Make upcoming milestone work across projects

Before: we took the next milestone due across all projects in the
search and found issues whose milestone title matched that
one. Problems:

1. The milestone could be closed.
2. Different projects have milestones with different schedules.
3. Different projects have milestones with different titles.
4. Different projects can have milestones with different schedules, but
   the _same_ title. That means we could show issues from a past
   milestone, or one that's far in the future.

After: gather the ID of the next milestone on each project we're looking
at, and find issues with those milestone IDs. Problems:

1. For a lot of projects, this can return a lot of IDs.
2. The SQL query has to be different between Postgres and MySQL, because
   MySQL is much more lenient with HAVING: as well as the columns
   appearing in GROUP BY or in aggregate clauses, MySQL allows them to
   appear in the SELECT list (un-aggregated).