Mask key comments when exposing SSH/Deploy Keys via the API

The GET SSH key API also shows the comments associated with the SSH key (if any) when this endpoint is used.

However, when we access the SSH key details via gitlab.com/username.keys, the comments in the keys are masked using the publishable_key method, but our API endpoint does no such masking.

We should bring parity to this, and mask the comments present in the SSH keys, when accessing SSH/Deploy keys via the API.