R1:acd955c6c9b9

Modularize application extensions to EditEngine

Summary:
Ref T9132. Currently, EditEngine had some branchy-`instanceof` code like this:

```
if ($object instanceof Whatever) {
do_magic();
}

if ($object instanceof SomethingElse) {
do_other_magic();
}
```

...where `Whatever` and `SomethingElse` are first-party applications like ProjectsInterface and SubscribersInterface.

This kind of code is generally bad because third-parties can't add new stuff, and it suggest something is kind of hacky in its…