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
added support for basic auth
moved notifier out of mailers directory

as suggested by @mrreynolds: this ain't a proper Rails mailer, the fact
that Naveed ends up sending e-mails is largely accidental in this
context
ensured notification service doesn't excessively block the response

eventually, we should separate Naveed requests from the controller flow
(e.g using ActiveJob)
improved idiomacy

as suggested by @mrreynolds
Revert "added support for basic auth"

the Authorization header is already being used for the bearer token,
thus Basic Auth doesn't make sense

This reverts commit 49f2d069e960b1412faec6d4f5d24baa544cbe8a.