[keyserver] Script to delete user

Summary: This script will let us ban spambots like Emilia: [ENG-9502](https://linear.app/comm/issue/ENG-9502/ban-emilia).

Test Plan:
I ran this in my local environment, but my user ID isn't 256 on staging, so I got this error:

```
[Error: user_is_not_staff] { code: 'GenericFailure' }
```

Once identity 0.41 is deployed to prod, I can try testing this in production.

Reviewers: varun

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13691

keyserver/src/scripts/delete-account.js

@@ -0,0 +1,15 @@
+// @flow
+
+import { main } from './utils.js';
+import { deleteAccount } from '../deleters/account-deleters.js';
+import { createScriptViewer } from '../session/scripts.js';
+import { privilegedDeleteUsers } from '../utils/identity-utils.js';
+
+async function deleteTargetAccount() {
+  const targetUserID = '';
+  await privilegedDeleteUsers([targetUserID]);
+  const viewer = createScriptViewer(targetUserID);
+  await deleteAccount(viewer);
+}
+
+main([deleteTargetAccount]);

keyserver/src/utils/identity-utils.js

@@ -23,4 +23,20 @@ async function findUserIdentities(
   );
 }
 
-export { findUserIdentities };
+async function privilegedDeleteUsers(
+  userIDs: $ReadOnlyArray<string>,
+): Promise<void> {
+  const [rustAPI, identityInfo, deviceID] = await Promise.all([
+    getRustAPI(),
+    verifyUserLoggedIn(),
+    getContentSigningKey(),
+  ]);
+  await rustAPI.privilegedDeleteUsers(
+    identityInfo.userId,
+    deviceID,
+    identityInfo.accessToken,
+    userIDs,
+  );
+}
+
+export { findUserIdentities, privilegedDeleteUsers };