Styling adjustments for .alignnone and .aligncenter

[kienstra]

Request For Code Review

Hi @delawski, Thanks a lot for your solution for the plugin issue #1109. Could you please review this, when you have a chance?

@westonruter noticed an issue with images have the class .alignnone.

https://v0-7-ampconfdemo.pantheonsite.io/2018/05/02/new-image-test-post/

They are horizontally stretched if they have a natural width less than the container.

Weston also pointed out that core themes don't have styling for .alignnone. This PR removes the selector for .alignnone, and also the width: 100% rule for .aligncenter.

Should these other rules for .alignnone also be removed? It looks like this width value is still causing the .alignnone image to be stretched.

Thanks, Piotr! You were a huge help today with the plugin issue.