Improve navigation

[tmfrnz]

we want to re-organise the content into logical sections and offer a shortcut for creating new content

• [x] top level sections: positions, stakeholders, outreach, my stuff
• [x] top level links: search, user account, more
• [x] introduce positions overview
• [x] introduce section navigation

[tmfrnz]

TODO

• [ ] section navigation should be inside scroll container (scroll out of view as currently implemented on Positions Overview
• [ ] clean up design/icons etc
  • [ ] waiting for icons
• [ ] "positions" should be active in main nav when viewing topics (list or single)