PLT-3754 EE: Add a Display Option to disable Join/Leave messages (#3808)

* PLT-3754 EE: Add a Display Option to disable Join/Leave messages

* Differentiate between join/leave add/remove messages

* Update user removed from channel text message

model/post.go

@@ -15,6 +15,7 @@ const (
 	POST_SLACK_ATTACHMENT      = "slack_attachment"
 	POST_SYSTEM_GENERIC        = "system_generic"
 	POST_JOIN_LEAVE            = "system_join_leave"
+	POST_ADD_REMOVE            = "system_add_remove"
 	POST_HEADER_CHANGE         = "system_header_change"
 	POST_CHANNEL_DELETED       = "system_channel_deleted"
 	POST_EPHEMERAL             = "system_ephemeral"
@@ -109,7 +110,7 @@ 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_SLACK_ATTACHMENT || o.Type == POST_HEADER_CHANGE) {
+	if !(o.Type == POST_DEFAULT || o.Type == POST_JOIN_LEAVE || o.Type == POST_ADD_REMOVE || o.Type == POST_SLACK_ATTACHMENT || o.Type == POST_HEADER_CHANGE) {
 		return NewLocAppError("Post.IsValid", "model.post.is_valid.type.app_error", nil, "id="+o.Type)
 	}