Honor auto_archive in the reminder email subject and the trial form

Two gaps found while checking whether auto_archive is honored
end-to-end (archive_overdue_clubs and the on-screen banner already
got it right):

- reminder_subject.txt branched only on notice.level, so a club with
  auto_archive off -- one that will NEVER be archived -- still got
  "Action required: X is about to be archived" as its subject line,
  contradicting the correctly-worded body underneath. Now gated on
  notice.level == 'error' AND notice.will_archive.
- TrialForm had no auto_renew/auto_archive fields at all, so a trial
  could only ever be started on the service defaults (both True).
  The only way to change either afterwards was "Change plan", which
  ends the trial as a side effect. Added both, matching
  SubscriptionForm's existing pair.

billing/templates/billing/email/reminder_subject.txt

@@ -1 +1 @@
-{% load i18n %}{% if notice.level == 'error' %}{% blocktrans with club=club.name %}Action required: {{ club }} is about to be archived{% endblocktrans %}{% else %}{% blocktrans with club=club.name %}Platform fees are due for {{ club }}{% endblocktrans %}{% endif %}
+{% load i18n %}{% if notice.level == 'error' and notice.will_archive %}{% blocktrans with club=club.name %}Action required: {{ club }} is about to be archived{% endblocktrans %}{% else %}{% blocktrans with club=club.name %}Platform fees are due for {{ club }}{% endblocktrans %}{% endif %}