Add system hook for ssh key changes

Enabling SSH Key create/delete notification in system hooks makes it possible to automatically push ssh keys to other systems so gitlab can be more tightly integrated with a development environment.

Code is self-explaining. It adds after_create and after_destroy hooks to the Key model and adds logic to process a Key update in the build_event_data method of the system hook service.
What do you think about moving this into a worker? Since it might take a little while for the hook to complete