Added system hooks messages for renaming and transferring a project

This is work in progress but I wanted to get some feedback on it in case there is something I've missed or a better way to do it.

As the title says when a project is renamed or transferred to another group a system hook message is generated.

Currently adds a couple of extra properties to the standard project system hooks messages.

old_path_with_namespace - The old path of the project so we can see what has been renamed / transferred changed_at - The time that change took place

My main concern is if there is a better way of doing this than passing the extra_data variable into the system hook. Would it be better to extend the project model with these 2 items as transient properties as I doubt they would need to be saved to the database? The other

I've also seen, but can't find, someone talking about the emails sent when a project is renamed or transferred doesn't reference the original project path so if you have similarly named projects then you might not know what has been moved. I think the extra_data (or however it is best to implement this) would allow the extra information to be sent in emails.

Closes System hooks for project transfer and repository rename