Go between post list / post content with keyboard

Closes #3940
- Pushing left/right changes the "focus" of the posts screen to the list / content, respectively
- Once one of these has been focused, up/down keyboards work on that section in particular
- By default, the post list is selected
- Using the keyboard to focus one of the two makes it go "poof" to let you know you changed stuff
Merge pull request #4285 from novaugust/read-draft-shortcuts

Go between post list / post content with keyboard