[MM-16389] Add icon_emoji parameter to webhooks (#11586)

[MM-16389] Add icon_emoji field to posts from incoming webhooks

To be used to substitute the profile picture on posts.

model/post.go

@@ -50,8 +50,11 @@ const (
 	POST_CUSTOM_TYPE_PREFIX     = "custom_"
 	POST_ME                     = "me"
 	PROPS_ADD_CHANNEL_MEMBER    = "add_channel_member"
-	POST_PROPS_ADDED_USER_ID    = "addedUserId"
-	POST_PROPS_DELETE_BY        = "deleteBy"
+
+	POST_PROPS_ADDED_USER_ID       = "addedUserId"
+	POST_PROPS_DELETE_BY           = "deleteBy"
+	POST_PROPS_OVERRIDE_ICON_URL   = "override_icon_url"
+	POST_PROPS_OVERRIDE_ICON_EMOJI = "override_icon_emoji"
 )
 
 type Post struct {