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

tests/app/main/views/test_send.py

@@ -611,7 +611,7 @@ def test_send_test_doesnt_show_file_contents(
     assert page.select('h1')[0].text.strip() == 'Preview of Two week reminder'
     assert len(page.select('table')) == 0
     assert len(page.select('.banner-dangerous')) == 0
-    assert page.select('input[type=submit]')[0]['value'].strip() == 'Send 1 text message'
+    assert page.select_one('button[type=submit]').text.strip() == 'Send 1 text message'
 
 
 @pytest.mark.parametrize('endpoint, template_mock, expected_recipient', [