AMP Stories should allow and make simple letterbox layout

[dvoytenko]

Describe the new feature or change to an existing feature you'd like to see

Stories should allow letterbox layout with proportional position/layout and font.

The proposed solution is to add this initially to the amp-story-grid-layout as:

<amp-story-grid-layout aspect-ratio="9:16">
</amp-story-grid-layout>

When specified, the size of the layer is set to be maximum allowed but preserving the specified aspect ratio. And the root font size is set a reference value that allows the content font to be assigned in proportionally to the layer's height.