[MJ-245]  prevent duplicate saved search urls

Prevents duplicate saved searches in both the API and in the django forms. Trying to save a duplicate URL through the API returns a 400 error. Trying to save in the forms generates a validation error in the form level (the model validation error wasn't sufficient because that's only triggered on save) and displays the error on the form.

I've refactored and modified the show errors method in the javascript to be more succinct and to show the more descriptive error message.

All tests pass.