Add edit/delete actions to the teams list

Admin-only, matching TeamCreateView/TeamUpdateView's existing gate.
Deleting cascades away the team's roster and staff assignments -- no
ProtectedError to catch, unlike a Member that orders/invoices can still
reference. Edit links to the detail page rather than the edit form
directly, same convention as the news and member lists.

management/context_processors.py

@@ -46,6 +46,7 @@ _NAV_SECTIONS = {
     "team_list": "team_list",
     "team_create": "team_list",
     "team_update": "team_list",
+    "team_delete": "team_list",
     "team_detail": "team_list",
     "roster_list": "roster_list",
     "staff_list": "staff_list",