RFC: Add subreddit to submission attributes

[TimJentzsch]

Currently, the subreddit can be extracted from the URL of a submission. However, you can't efficiently run queries depending on the subreddit. This would be cool for buttercup stats (which subreddit do I transcribe the most?), but also for the engagement team to determine which subreddits are popular among the volunteers. This would be helpful to select future partner subreddits.

I'm not yet sure what the best way would be to integrate this. Adding a subreddit attribute is probably not the way to go since we want to stay independant of Reddit.
One idea would be to add a source entry for every subreddit and then link to that, instead of a generic reddit source.