Explain "optionally GPG encrypt w/public key" better

I’ve found the existing footer text a bit confusing — "of course I want all the security!" would be most users’ reaction — since most people directed to SecureDrop at most media organizations won’t be GPG-savvy.

And since SD encrypts-on-receipt, we should let users know that since otherwise they might be scared that the files are unencrypted unless they take this optional GPG step.

I’ve also added a warning to the "why encrypt with public key" page about signing the encrypted file & how that reveals your keyid. (This has happened once or twice.)