Fix potential form ID discrepancy between search form text input and button

[cmsetzer]

In #3659 we modified the search_form.html template so the search text input can point to an arbitrary form. (This is useful on the paid registrar approval page, because there are two forms that submit data using different methods.)

I missed one detail: if an arbitrary form_id is supplied, the search submission button needs to point to it as well. Otherwise, hitting Enter within the text input and clicking the submit button may produce unexpectedly different results.