Performance: Inbox Paging / loading (#11157)

* load messages per conversation

* only sort ones in ui

*  add contributor to message

* fix correct message layout/message

* mugenScroll on chatMessages

* fix lint, no mugen-scroll, use own scroll handler

* fix height / margin of modal + use button to load more

* fix tests

* user data from inbox

* style "load earlier messages"

*  move mapMessage to the inbox api result / extract sentMessage of members-api-controller

* fix test back

* fix test

* keep last scroll position

* just set the Id of the returned message instead of all other properties

* fix add new messages (buttons were hidden) + load more

* item-mounted debounce to trigger the re-scrolling

test/api/v4/members/POST-flag_private_message.test.js

@@ -22,7 +22,7 @@ describe('POST /members/flag-private-message/:messageId', () => {
     let senderMessages = await userToSendMessage.get('/inbox/messages');
 
     let sendersMessageInSendersInbox = _.find(senderMessages, (message) => {
-      return message.uuid === receiver._id && message.text === messageToSend;
+      return message.toUUID === receiver._id && message.text === messageToSend;
     });
 
     expect(sendersMessageInSendersInbox).to.exist;