(Bug 4196) New comments by replying to comment notifications

* Refactors the existing LJ::Emailpost into
  DW::EmailPost/DW::EmailPost::Base/DW::EmailPost::Entry/DW::EmailPost::Comment

* DW::EmailPost::Entry works by providing a PIN, same as before

* DW::EmailPost::Comment works by having a one-time auth per
  comment/comment thread, not time-limited, automatically generated by
  DW

* comment notification email has a Reply-To header, and additional
  instructions. Instead of going through the comment form, simply
  "reply" via the email client.