Fix bug where old leader signups displayed active

In short, the current system will accept a leader's signup, but then
even if that person is *removed* from the collection of a trip's
leaders, they will remain present in the total leader count (notably,
where the leader count itself is inaccurate)!

There are other parts of that issue that remain in need of a solution,
but this at least corrects the error of miscategorizing leader signups.

ws/templates/for_templatetags/view_trip.html

@@ -22,9 +22,20 @@
 
   {# The leader table is so participants can see leader notes #}
   {#   (or so leaders can see who's driving on other trips)   #}
-  {% if has_notes and signups.leader or viewing_participant.is_leader and trip.leaders.count %}
+  {% if has_notes and signups.leaders_on_trip or viewing_participant.is_leader and trip.leaders.count %}
     <h3>Leaders ({{ trip.leaders.count }})</h3>
-    {% signup_table signups.leader has_notes show_drivers=viewing_participant.is_leader all_participants=trip.leaders%}
+    {% signup_table signups.leaders_on_trip has_notes show_drivers=viewing_participant.is_leader all_participants=trip.leaders %}
+  {% endif %}
+
+  {# A list of previous leaders who signed up is useful to #}
+  {# - Any individuals in the list (who may be confused about their status) #}
+  {# - Trip creator, or other leaders curious about headcount #}
+  {% if has_notes and signups.leaders_off_trip %}
+    <h3>Leader signups, no longer on trip</h3>
+    <div class="alert alert-info">
+      These individuals previously signed up as leaders, but are no longer leaders on the trip.
+    </div>
+    {% signup_table signups.leaders_off_trip has_notes show_drivers=viewing_participant.is_leader %}
   {% endif %}
 
   {% if signups.on_trip %}

ws/templatetags/trip_tags.py

@@ -133,6 +133,8 @@ def get_signups(model=models.SignUp):
         signups = signups.select_related('participant', 'trip')
         return signups.select_related('participant__lotteryinfo')
 
+    trip_leaders = trip.leaders.all()
+    leader_signups = get_signups(models.LeaderSignUp)
     context = {
         'trip': trip,
         'is_trip_leader': perm_utils.leader_on_trip(participant, trip),
@@ -149,7 +151,10 @@ def get_signups(model=models.SignUp):
         'waitlist': wl_signups,
         'off_trip': signups.filter(on_trip=False).exclude(pk__in=wl_signups),
         'on_trip': signups.filter(on_trip=True),
-        'leader': get_signups(models.LeaderSignUp),
+        'leaders_on_trip': [s for s in leader_signups if s.participant in trip_leaders],
+        'leaders_off_trip': [
+            s for s in leader_signups if s.participant not in trip_leaders
+        ],
     }
     context['has_notes'] = (
         bool(trip.notes)