Public/personalized department overview

[dpausp]

The departments view (/departments) is publicly accessible at the moment but hidden. It's not very useful for normal users as it only displays the names of departments.

• change path to /d
• Add a link in the top navbar to the departments view
• department overview should be split between "my departments" and "other departments" for logged-in users
• departments should show
  • links to drafts, all propositions (without drafts)
  • current voting phases
  • number of members