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
Encourage would be to strong a word, for sure. You can still override the behavior by registering a different handler, so I was vivisector with this solution. It's also sort of wrong to unregister one without unregistering the other anyways.