When translating a `<button>` element's contents, it is tedious  to make
the translation text available to a block scope.

For instance, when rendering a `<button type="submit">` with an SVG
element as its child, passing translated label text to that SVG
element's [`<title>`][svg-title] element requires an extra call to
`I18n.translate`.

Prior to this commit, doing so would require a double lookup of the
translation key:

```erb
<%# one time here, implicitly %>
<%= form.button do %>
  <svg>
    <title>
      <!-- one time here, explicitly -->
      <%= translate("helpers.submit.post.create") %>
    </title>
    <!-- ... -->
  </svg>
<% end %>
```

This commit modifies the `ActionView::Helpers::FormBuilder#button` to
check for invocations that are passed a block, and conditionally yield
the contents of `submit_default_value` as the argument.

The new view code might look something like this:

```erb
<%= form.button do |text| %>
  <svg>
    <title><%= text %></title>
    <!-- ... -->
  </svg>
<% end %>
```

Callers of the helper are still free to omit the block parameter.

[svg-title]: https://developer.mozilla.org/en-US/docs/Web/SVG/Element/title