Fix admin search slow

- Implement new AdvancedAdminSearchMixin
- Don't convert every value to text for search, so DB indexes can be used
- Change admin search from `iexact (=)`  to `exact (==)`

safe_transaction_service/utils/admin.py

@@ -25,6 +25,7 @@ def queryset(self, request, queryset):
             return queryset
 
 
+# TODO Use the class in safe-eth-py
 class AdvancedAdminSearchMixin:
     """
     Use database indexes when using exact search instead
@@ -40,9 +41,6 @@ def get_search_results(self, request, queryset, search_term):
         to allow `exact` search that uses database indexes
         """
 
-        # Apply keyword searches.
-        print("HOLA")
-
         def construct_search(field_name):
             if field_name.startswith("^"):
                 return "%s__istartswith" % field_name[1:]