Add which subreddit a post belongs to in FeedsViewController

[RJ-Clegg]

Is your feature request related to a problem? Please describe.
Whilst browsing the feeds, it would be helpful to know which subreddit a particular post belongs to

Describe the solution you'd like
Perhaps a small label at the top (left or right) of each ListingTableViewCell in the FeedViewController showing which subreddit the post belongs to - perhaps also in a different colour?

Describe alternatives you've considered
Not sure about alternatives to this feature; the only place for it is in each ListingTableViewCell

Additional context
Since every post (Listing) has to belong to a subreddit; there should be no logic to hide this label and therefore it should be easy enough to add above the top level UIStackView in that cell. However I would perhaps wrap it in, it's own UIStackView set to horizontal alignment. This way, should we add new labels to the top of the cell, we won't have to mess about with AutoLayout constraints directly.