Add information security policy acknowledgement to onboarding

Created by: dadlerj

This is only an initial outline of what we'll ultimately need in terms of new hire information security acknowledgement. While this will work for the near future, I'd love to ask for support from the @sourcegraph/security team in fleshing out an actual internal policy doc that is actually useful and relevant, while still being concise and avoiding a bunch of boilerplate.

Please let me know if you support the form I've added here for now!