Publicize: add Reddit

Adding functionality for posting directly to Reddit on publish like Twitter and Facebook. Obviously, to avoid spam, this should also be limited to posting to a single Subreddit.

Start by locating the existing Twitter and Facebook integrations used when publishing, then trace their publish entry points and configuration. Add Reddit posting with a single-subreddit limit, and verify that publishing sends to the configured subreddit without enabling broader posting.

Written by the indexing model from the issue text.