Discussion tab should be separate from tree and roadmap tabs

Problem

From #12765 (comment 229609751)

There is no Discussion tab anymore, leading all comments and system notes to be rendered in the Epics and Issues tab. This also creates some weird behavior when switching between Epics and Issues to Roadmap.

Epic_tree

Proposal

1. The tabs should stop at the bottom of the tree/roadmap tab areas (similar to how the diff file headers stop at the bottom of each diff file)

2. Can the discussion exist independently so that the width doesn't change depending on whether you're on Epics and Issues or Roadmap?