Add key bindings to open subreddit and user page

I ported yet another open rtv pull request: https://github.com/michael-lazar/rtv/pull/688

Like the previous two, this too is all work by @aexl (aka https://github.com/goggle), I merely did the merge and resolved the conflicts. Git history should be preserved accurately with this merge.

Original description:

As described in #687 it would be nice to have the ability to open the subreddit of the currently selected item. This PR adds this functionality as well as another keybinding to open the authors user page of the currently selected item.