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_api_keys.py

@@ -242,7 +242,8 @@ def test_should_show_confirm_revoke_api_key(
     )
     assert normalize_spaces(page.select('.banner-dangerous')[0].text) == (
         'Are you sure you want to revoke this API key? '
-        '‘some key name’ will no longer let you connect to GOV.UK Notify.'
+        '‘some key name’ will no longer let you connect to GOV.UK Notify. '
+        'Confirm'
     )
     assert mock_get_api_keys.call_args_list == [
         call(