Add reminder to not paste private SSH keys

## What does this MR do?

Just got the "How would you make GitLab better?" mail and thought that I could follow it.

This commit adds a short reminder to not paste the private part of an SSH key to the form where one can add SSH keys to one's account. I think that's an useful message, both for people who aren't that experienced yet and for people who are sleep-deprived. :-)

I decided to just list the most common key type, `ssh-rsa`. The full list of key types in the [CVS repository of OpenSSH](http://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/src/usr.bin/ssh/sshkey.c?rev=HEAD&content-type=text/plain) contains some types which I'd regard as a bit obscure. Mentioning all of those types would probably clutter the form too much. We could think about whether to list what is probably the second most common key type, namely `ssh-dss`. But note that this key type [seems to be deprecated](https://security.stackexchange.com/questions/5096/rsa-vs-dsa-for-ssh-authentication-keys).

## Are there points in the code the reviewer needs to double check?

I didn't actually test this one-line change.

## Why was this MR needed?

It's not, strictly speaking, needed, but could be considered a nice addition.

See merge request !4399