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.
All system hooks are already being executed via a worker if I read the code right. `execute_hooks_for` calls `execute_hooks` calls `async_execute_hook`.