- Add missing types in src/annotator and src/boot.

 - Simplify the return types of some config parsing functions by
   coercing invalid values to `null` or some other default. This reduces
   complexity with the types elsewhere and also potentially avoids some
   confusing errors if invalid values are specified here.

 - Add type definitions for hammerjs, scroll-into-view and
   lodash.debounce. The definitions for hammerjs and scroll-into-view
   come from DefinitelyTyped. For lodash a minimal custom definition has
   been written because the @types/lodash.debounce package includes
   types for the whole of lodash.

   In order to use these types the `allowSyntheticDefaultImports` flag
   had to be enabled in the TS configs.

   A consequence of adding types for scroll-into-view is that some
   missing null checks in sidebar components were found.