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/test_conversation.py

@@ -1,4 +1,5 @@
 import json
+import re
 from datetime import datetime
 from unittest.mock import Mock
 
@@ -265,7 +266,7 @@ def test_conversation_reply_shows_link_to_add_templates_if_service_has_no_templa
         notification_id=fake_uuid,
     )
     page_text = page.find('p', class_='bottom-gutter').text
-    link = page.find('a', text='Add a new template')['href']
+    link = page.find('a', text=re.compile('Add a new template'))['href']
 
     assert normalize_spaces(page_text) == 'You need a template before you can send text messages.'
     assert link == url_for(