Changing HACKING.md to simplify and update
delete obsolete documentation files
remove misc. refs to deaddrop
remove refs. to deaddrop, rewrite for newest config defaults, and reformat
remove ref to deaddrop, and fake email from uid

Since the reference to deaddrop was part of the fake email address, it made
sense to remove them both since the fake email was not actually needed.
I modified getkey so it should still work, even if used with reply keypairs
generated with previous versions of securedrop (they'll still have the
hashed codename in their uid).
update application README