Add progressive enhancement

When someone selects that they are accepting the agreement on behalf of
someone else then they need to provide that person’s details. Otherwise
they shouldn’t care about these extra fields.

This commit uses the progressive disclosure pattern from the GOV.UK
Frontend Toolkit to hide the additional fields unless someone selects
the relevant radio button.

tests/app/main/views/test_agreement.py

@@ -133,11 +133,19 @@ def test_show_accept_agreement_page(
     )
     assert page.select('input[name=who]')[0]['value'] == 'me'
     assert 'checked' not in page.select('input[name=who]')[0]
+    assert 'data-target' not in page.select('.multiple-choice')[0]
     assert normalize_spaces(page.select_one('label[for=who-1]').text) == (
         'I’m accepting the agreement on behalf of someone else'
     )
     assert page.select('input[name=who]')[1]['value'] == 'someone-else'
     assert 'checked' not in page.select('input[name=who]')[1]
+    assert page.select('.multiple-choice')[1]['data-target'] == 'on-behalf-of'
+    assert [
+        field['name']
+        for field in page.select('#on-behalf-of.conditional-radio-panel input')
+    ] == [
+        'on_behalf_of_name', 'on_behalf_of_email'
+    ]
 
     assert normalize_spaces(page.select_one('label[for=on_behalf_of_name]').text) == (
         'Who are you accepting the agreement on behalf of?'