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

model/post.go

@@ -14,8 +14,12 @@ const (
 	POST_DEFAULT               = ""
 	POST_SLACK_ATTACHMENT      = "slack_attachment"
 	POST_SYSTEM_GENERIC        = "system_generic"
-	POST_JOIN_LEAVE            = "system_join_leave"
-	POST_ADD_REMOVE            = "system_add_remove"
+	POST_JOIN_LEAVE            = "system_join_leave" // Deprecated, use POST_JOIN_CHANNEL or POST_LEAVE_CHANNEL instead
+	POST_JOIN_CHANNEL          = "system_join_channel"
+	POST_LEAVE_CHANNEL         = "system_leave_channel"
+	POST_ADD_REMOVE            = "system_add_remove" // Deprecated, use POST_ADD_TO_CHANNEL or POST_REMOVE_FROM_CHANNEL instead
+	POST_ADD_TO_CHANNEL        = "system_add_to_channel"
+	POST_REMOVE_FROM_CHANNEL   = "system_remove_from_channel"
 	POST_HEADER_CHANGE         = "system_header_change"
 	POST_DISPLAYNAME_CHANGE    = "system_displayname_change"
 	POST_PURPOSE_CHANGE        = "system_purpose_change"
@@ -121,6 +125,8 @@ func (o *Post) IsValid() *AppError {
 
 	// should be removed once more message types are supported
 	if !(o.Type == POST_DEFAULT || o.Type == POST_JOIN_LEAVE || o.Type == POST_ADD_REMOVE ||
+		o.Type == POST_JOIN_CHANNEL || o.Type == POST_LEAVE_CHANNEL ||
+		o.Type == POST_REMOVE_FROM_CHANNEL || o.Type == POST_ADD_TO_CHANNEL ||
 		o.Type == POST_SLACK_ATTACHMENT || o.Type == POST_HEADER_CHANGE || o.Type == POST_PURPOSE_CHANGE ||
 		o.Type == POST_DISPLAYNAME_CHANGE || o.Type == POST_CHANNEL_DELETED) {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.type.app_error", nil, "id="+o.Type)