Add styles for Recent Comments widget

[westonruter]

The Recent Comments widget outputs the following style element in the head via \WP_Widget_Recent_Comments::recent_comments_style():

<style type="text/css">.recentcomments a{display:inline !important;padding:0 !important;margin:0 !important;}</style>

The show_recent_comments_widget_style filter allows this to be turned off, which it must be for AMP validity. Since it is off, the theme itself will need to incorporate this CSS rule, except for !important since this is also invalid in AMP.

Ref. ampproject/amp-wp#864

[kopepasah]

Ready for review in #54.

[kienstra]

Steps To Test

1. Go to a test page with a 'Recent Comments' widget
2. Verify that the comments display
3. Verify that clicking them leads to a post

[csossi]

@kienstra , Could there be some sort of cue for user to resolve error re: URL format here?

[westonruter]

Yes, there could. I think if you submit this as-is, however, AMP there will be an error displayed at that time, will it not? In this case here, the field is marked as url and so the browser is reporting it as invalid. However, WordPress on the backend will correctly prepend it with http:// so no error will ensue. We could add more error reporting via https://www.ampproject.org/docs/reference/components/amp-form#custom-validations however I think this is beyond the scope of just adding commenting support. WordPress itself doesn't provide contextual error reporting if you supply a bad URL.

[csossi]

understood, @westonruter - noting this as verified in QA - cheers