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

api/channel.go

@@ -4,7 +4,6 @@
 package api
 
 import (
-	"fmt"
 	"net/http"
 	"strconv"
 	"strings"
@@ -644,7 +643,7 @@ func addMember(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	go app.PostUserAddRemoveMessage(c.Session.UserId, channel.Id, channel.TeamId, fmt.Sprintf(utils.T("api.channel.add_member.added"), nUser.Username, oUser.Username), model.POST_ADD_REMOVE)
+	go app.PostAddToChannelMessage(oUser, nUser, channel)
 
 	app.UpdateChannelLastViewedAt([]string{id}, oUser.Id)
 	w.Write([]byte(cm.ToJson()))
@@ -697,7 +696,7 @@ func removeMember(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	go app.PostUserAddRemoveMessage(c.Session.UserId, channel.Id, channel.TeamId, fmt.Sprintf(utils.T("api.channel.remove_member.removed"), user.Username), model.POST_ADD_REMOVE)
+	go app.PostRemoveFromChannelMessage(c.Session.UserId, user, channel)
 
 	result := make(map[string]string)
 	result["channel_id"] = channel.Id