Replace straightforward buttons and links styled like buttons

This replaces the buttons that aren't part of a macro and that we don't
need to write additional styles for with their govuk-frontend equivalent.

There were some links that were styled to look like buttons, so these
have also been replaced with the new govuk-frontend macro.

There was one button on the `choose-account.html` template that was in a
section of code that was never reached - this has been deleted.

tests/app/main/views/organisations/test_organisation.py

@@ -50,7 +50,7 @@ def test_organisation_page_shows_all_organisations(
             expected_hints[index]
         )
     assert normalize_spaces(
-        page.select_one('a.button-secondary').text
+        page.select_one('a.govuk-button--secondary').text
     ) == 'New organisation'
     get_organisations.assert_called_once_with()