feature(webapp): Add support for a basic WebApp Manifest file

Fixes #7493


TODO:
 - [x] Choose a component. i.e. `feature(???)`
 - [x] Actually register the `init()` callback with Elgg
 - [x] Remove the `@flow` annotation
Entities are so deeply coupled with a mountain of functionality that having them as a dependency makes it very easy for future devs to pull too much of them. I guess it feels like injecting a service locator and the associated ills.