Window object augmentations are not typed

[ianyong]

The application extends the global window object with additional properties, most of which can be found in public/externalLibs/. However, these augmentations are not typed. The current practice is to cast the window object as any in order to access these additional properties. (Simply search for window as any in the codebase to find all such instances.)

In order to make such accesses type-safe, we can make use of TypeScript's ability to augment global types.