Use <button> not <input>s for form submission

Both `<button type='submit'>Submit<button>` and
`<input type='submit' value='Submit'>` can be used to submit a form.

We have historically[1] used `<input>` because it’s better-supported by
IE6 in that:
- the `submit` attribute is mandatory on `<button>`, not on `<input>`
- the `innerHTML` of a button will be submitted to the server, not the
  value (as in other browsers)

Reasons to now use `<button>` instead:
- IE6/7 support is no longer a concern (especially with deprecation of
  TLS 1.0 on the way)
- Because an `<input>` element can’t have children, the pseudo-element
  hack[2] used to ensure the top edge of the button is clickable doesn’t
  work. We’re seeing this bug[3] affect real users in research.

1. We inhereted our buttons from Digital Marketplace, here is me making
   that change in their code:  8df7e2e79e (diff-b1420f7b7a25657d849edf90a70ef541)
2. 24e1906c0d (diff-ef0e4eb6f1e90b44b0c3fe39dce274a4R79)

3. https://github.com/alphagov/govuk_elements/issues/545

app/templates/views/notifications/check.html

@@ -51,7 +51,7 @@
       <input type="hidden" name="csrf_token" value="{{ csrf_token() }}" />
       {% if not error %}
         {% if template.template_type != 'letter' or not request.args.from_test %}
-        <input type="submit" class="button" value="Send 1 {{ message_count_label(1, template.template_type, suffix='') }}" />
+        <button type="submit" class="button">Send 1 {{ message_count_label(1, template.template_type, suffix='') }}</button>
         {% else %}
           <a href="{{ url_for('main.check_messages_preview', service_id=current_service.id, template_type=template.template_type, upload_id=upload_id, filetype='pdf') }}" download class="button">Download as a printable PDF</a>
         {% endif %}