Inbox: Add API to list conversations (#11110)

* Add API to list inbox conversations

* fix test + add api doc

* use `.lean()`

* orderBy after the the grouped conversations are loaded

* fix ordering

test/api/v4/inbox/GET-inbox-conversations.test.js

@@ -0,0 +1,44 @@
+import {
+  generateUser,
+} from '../../../helpers/api-integration/v4';
+
+describe('GET /inbox/conversations', () => {
+  let user;
+  let otherUser;
+  let thirdUser;
+
+  before(async () => {
+    [user, otherUser, thirdUser] = await Promise.all([generateUser(), generateUser(), generateUser()]);
+
+    await otherUser.post('/members/send-private-message', {
+      toUserId: user.id,
+      message: 'first',
+    });
+    await user.post('/members/send-private-message', {
+      toUserId: otherUser.id,
+      message: 'second',
+    });
+    await user.post('/members/send-private-message', {
+      toUserId: thirdUser.id,
+      message: 'third',
+    });
+    await otherUser.post('/members/send-private-message', {
+      toUserId: user.id,
+      message: 'fourth',
+    });
+
+    // message to yourself
+    await user.post('/members/send-private-message', {
+      toUserId: user.id,
+      message: 'fifth',
+    });
+  });
+
+  it('returns the conversations', async () => {
+    const result = await user.get('/inbox/conversations');
+
+    expect(result.length).to.be.equal(3);
+    expect(result[0].user).to.be.equal(user.profile.name);
+    expect(result[0].username).to.be.equal(user.auth.local.username);
+  });
+});