implemented comment notifications

when a new comment is added, a notification is sent to the respective
post's author and to all previous commenters

using Naveed (https://github.com/innoq/naveed) to notify users via
e-mail - the primary advantage being that Naveed allows users to
customize their preferences

caveats:
* only the original comment triggers a notification, any subsequent
  edits are ignored here (the assumption being that edits are mostly
  minor fixes)
* configuration resides in undocumented environment variables
  (crucially, `IBLOG_NAVEED_URL` needs to point to `/outbox`)
* `url_for` probably won't work as expected, as the deployed application
  likely doesn't know its own hostname
* users are only identified via their handle rather than their full name
  (though that's the case throughout the entire application anyway)

regarding Naveed's API: the proper RESTful way would be to go full
HATEOAS, retrieving the /outbox endpoint via hypermedia traversal -
however, then we'd still have to find a way to make Naveed tell the
client about the expected form fields' semantics