Merge "supporter" and "supporters" apps

It's a little confusing having both a "supporter" and "supporters" app. This is
a fairly minor change to move the sustainer banner urls/views into the existing
"supporters" app. The functionality isn't directly related, but it is all about
sustainers.

conservancy/urls.py

@@ -43,7 +43,7 @@ urlpatterns = [
     path('news/', include('conservancy.news.urls')),
     path('sponsors/', sponsors.view),
     path('sponsors/index.html', sponsors.view),
-    path('sustainer/', include('conservancy.supporter.urls')),
+    path('sustainer/', include('conservancy.supporters.urls')),
     path('usethesource/', include('conservancy.usethesource.urls')),
 
     # Directories of templates and files