The app originated from iOS and the design language of swiping from left edge in is always to go back in the history; and design language of swiping from right edge in is always to go forward in the history stack. The forward swipe is not straight forward in the ionic framework the app is currently implemented in, but in all cases it shouldn’t break the design language.
The design language generally suggests something near the header (i.e. the sort and moderator icons area ), inset from the existing interaction button, to have up/down arrows to move to previous/next item in the same list, and I think maybe some optional buttons there might make sense.
Though, given the type of app this is (social link aggregator with up/down votes), simple arrows might be confusing for some users. Perhaps a visually different set of arrows might do the trick…?