Insufficient redaction for environment dumps

[zackw]

The environment dump you get when there's an internal error isn't being redacted thoroughly enough. These items appear to contain secrets, and should be entirely redacted:

• SECRET_KEY_BASE
• action_dispatch.request.unsigned_session_cookie
• action_dispatch.secret_key_base
• action_dispatch.secret_token
• rack.request.cookie_hash
• rack.request.cookie_string

The :exception_recipients field of exception_notifier.options should also be redacted, as it exposes people's email addresses.