Summary:
in Layout plugin, scrollbars where often not visible, for example to see the vertical scrollbar, one had to scroll to the horizontal end first.
Also introduced the `Scrollable` component to simplify this in the feature and separate the concepts of rendering something large and making it scrollable.
This diff cleans up the layout structure and fixes the problem
changelog: Fixed several minor layout issues in the Layout plugin
Reviewed By: cekkaewnumchai
Differential Revision: D21283157
fbshipit-source-id: 81849151475165796c65001616f038a9d6cbdfb2
b0ab9b9b98