Fold Roster and Staff into Teams, with a per-season switcher and real CRUD

One "Teams" nav entry now, matching how Family has none of its own and
is only reached through Members -- Roster/Staff were pure club-wide
list stubs with no way to add/remove anyone except Django admin.
Opening a team shows its roster and staff for whichever season is
selected (?season=, defaulting to the current one, same pattern as the
Memberships page); adding/editing/removing either is gated to that
team's own manager or a club admin (TeamManagerRequiredMixin, defined
earlier but never actually wired up until now).

management/context_processors.py

@@ -48,8 +48,12 @@ _NAV_SECTIONS = {
     "team_update": "team_list",
     "team_delete": "team_list",
     "team_detail": "team_list",
-    "roster_list": "roster_list",
-    "staff_list": "staff_list",
+    "team_roster_add": "team_list",
+    "team_roster_update": "team_list",
+    "team_roster_remove": "team_list",
+    "team_staff_add": "team_list",
+    "team_staff_update": "team_list",
+    "team_staff_remove": "team_list",
     "news_list": "news_list",
     "news_create": "news_list",
     "news_detail": "news_list",