This change ports the base PageView class to spina and TypeScript. As

part of this, the code has been moved into the common subdirectory to

match the bundle that it's a part of.
The one change that was required outside of this class was to update the

DiffViewerPageModel to use preinitialize instead of constructor.

The use of the reserved name constructor has bitten us in the past,

and wasn't working anymore now that the base class was an ES6 class.

Ran js-tests.
Verified that page views still rendered correctly.