perf(finance/admin): remove `is_valid` from list view (#9)

The new unified statement view is very slow in production. This is
caused by the computation of `is_valid` for every entry of the list,
which entails at least one database query per row. Instead of removing
the `is_valid` field from the list view, we could prefetch the related
fields, but I don't think the `is_valid` provides any noticeable benefit
anyway.

We also add pagination to the statement view.

jdav_web/finance/admin.py

@@ -65,11 +65,12 @@ def decorate_statement_view(model, perm=None):
 @admin.register(Statement)
 class StatementAdmin(CommonAdminMixin, admin.ModelAdmin):
     fields = ['short_description', 'explanation', 'excursion', 'status']
-    list_display = ['__str__', 'total_pretty', 'created_by', 'submitted_date', 'is_valid', 'status_badge']
+    list_display = ['__str__', 'total_pretty', 'created_by', 'submitted_date', 'status_badge']
     list_filter = ['status']
     search_fields = ('excursion__name', 'short_description')
     ordering = ['-submitted_date']
     inlines = [BillOnStatementInline]
+    list_per_page = 25
 
     def has_change_permission(self, request, obj=None):
         if obj is None: