Investigate using multireddits to fetch posts for queue

[TimJentzsch]

You can concatenate multiple subreddits with a + to get the feed of all subreddits combined:
https://www.reddit.com/r/me_irl+me_irlgbt+thatHappened+therewasanattempt
Supposedly, this also works for the API.

This could significantly reduce the number of API requests we need to fetch new posts for the queue.
We should investigate if we can use this technique.

Note that there is supposed to be a maximum limit of subs you can have in a single multireddit, but it's supposed to be a lot (I'm not sure what the limit is).
So we might have to batch them into multiple calls, but it should still be a lot less than we need right now.