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 architecture. Ideally, we would eventually get rid of almost all of this.
T9789 is a similar discussion of this for the next layer down (`TransactionEditor`) and plans to get rid of branchy-instanceofs there too.
Since I'm about to add more stuff here (for Custom Fields), split it out first so I'm not digging us any deeper than I already dug us.
Broadly, this allows third-party extensions to add fields to every EditEngine UI if they want, like we do for Policies, Subscribers, Projects and Comments today (and CustomFields soon).
Test Plan:
{F1007575}
- Observed that all fields still appear on the form and seem to work correctly.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9132
Differential Revision: https://secure.phabricator.com/D14599
