PLT-5225 Added separate post types for certain system messages (#5193)

* Added separate post types for system_join_leave and system_add_remove posts

* Added username prop to channel deleted posts

* Changed slack import to properly generate the new join/leave messages

* Added username prop to channel update posts

* Moved Post(AddTo/RemoveFrom)ChanneMessage into app package

app/slackimport.go

@@ -307,12 +307,23 @@ func SlackAddPosts(teamId string, channel *model.Channel, posts []SlackPost, use
 				l4g.Debug(utils.T("api.slackimport.slack_add_posts.user_no_exists.debug"), sPost.User)
 				continue
 			}
+
+			var postType string
+			if sPost.SubType == "channel_join" {
+				postType = model.POST_JOIN_CHANNEL
+			} else {
+				postType = model.POST_LEAVE_CHANNEL
+			}
+
 			newPost := model.Post{
 				UserId:    users[sPost.User].Id,
 				ChannelId: channel.Id,
 				Message:   sPost.Text,
 				CreateAt:  SlackConvertTimeStamp(sPost.TimeStamp),
-				Type:      model.POST_JOIN_LEAVE,
+				Type:      postType,
+				Props: model.StringInterface{
+					"username": users[sPost.User].Username,
+				},
 			}
 			ImportPost(&newPost)
 		case sPost.Type == "message" && sPost.SubType == "me_message":