Loading child messages when opening the TMF

Part of #2202 (closed)

Purpose of this MR is to be able to load child messages when opening TMF.

What has been done:

• covered the chatService with more tests
• converted findChatMessagesForTroupe to async-await
• covered the apiClient with some tests
• new function chatService.findThreadChatMessages
• exposing the child messages on /api/v1/rooms/{roomId}/chatMessages/{parentId}/thread
• fetching child messages upon opening TMF
• style Loading and error messages
• log error if there is one during loading child messages (done by apiClient)
• if we proceed with using the VuexApiRequest:
  • write tests
  • in a separate MR: convert the left menu code to use it
• make sure that integration of VuexApiRequest with the store is tested

UX reference

Error	Loading	Success

Testing

The threaded-conversations toggle should be on. http://localhost:5000/api_web/features/threaded-conversations/1

1. Open a thread message feed
2. create 2 messages using the TMF chat input
3. refresh the page and start a thread from the same parent message
4. you should see the messages being fetched

Index:

1. I've tested that the schema change results in the correct index being created: parentId_1_sent_-1

   ChatMessageSchema.index(
     { parentId: 1, sent: -1 },
     {
       partialFilterExpression: { parentId: { $exists: true } },
       background: true
     }
   );

   results in:

   troupeSet:PRIMARY> db.chatmessages.getIndexes()
   [
   ...
   {
   	"v" : 1,
   	"key" : {
   		"parentId" : 1,
   		"sent" : -1
   	},
   	"name" : "parentId_1_sent_-1",
   	"ns" : "gitter.chatmessages",
   	"partialFilterExpression" : {
   		"parentId" : {
   			"$exists" : true
   		}
   	},
   	"background" : true
   }
   ]