implemented comment notifications

this triggers e-mail notifications via [Naveed](https://github.com/innoq/naveed) when a new comment is added

a notification is sent to the respective post's author and to all previous commenters - i.e. there's currently no support for @-mentions

the primary advantage of using Naveed is that it allows users to customize their preferences by muting specific apps (i.e. granularity is limited to sources, not individual threads)

note that credentials are supplied via environment variables - otherwise notifications fail silently

also note that `url_for` probably won't work as expected since the application doesn't know its own hostname

more details in the commit message

eventually we might consider delegation notification triggers to a separate app that consumes the RSS feed, but for now this is the pragmatic approach