Better Thunderbird/Enigmail example screenshots

[micahflee]

I got feedback about the 'Encrypting, Decrypting, and Signatures' subsection in the '"Pretty Good Privacy" (PGP) Email Encryption' section.

In the email example on pp.18-20, the discussion would be clearer if the screenshot email weren't from and to you. For newcomers to public-key encryption, there is often confusion about my/your public/private keys.

I think this makes a lot of sense, and should take the time to set up new email addresses and keys to explain how it works better.

[runasand]

We should also encourage users of Enigmail to enable the setting that forces a confirmation dialog when sending non-encrypted email.