Add default stylesheet for AMP content when in theme support mode

[westonruter]

As discovered in #921 (comment) there are some CSS rules that the sanitizer expects to be present which are not currently being output when there is theme support. In particular, these styles seem to be necessary:

https://github.com/Automattic/amp-wp/blob/7b88ae0acd450e1658f1248d11319a01e7c5e920/templates/style.php#L39-L49

An audit needs to be done to see which styles are needing to be present in the default styles, without requiring each theme to implement these on their own. Given that the plugin will automatically concatenate CSS into the style[amp-custom] element, a new CSS file can be added and enqueued normally when an AMP page is rendered in theme support.

Note that the CSS enqueued in theme support should not include any of the styles that are used in the legacy paired mode templates.