Add themes
Created by: njoyard
Done in this PR
- Add a new "theme" model with related links
- Add many to many relationships with Proposal, Dossier and Position
- Add theme to the admin
- Remove tags from Position
- Add a page to list themes and show a theme with details (description, links, dossiers, proposals, positions)
TODO but in a future PR (after v3 design merge, no need to code things that will be destroyed tomorrow):
- add theme(s) selection on public position form
- show themes as tag badges everywhere
- allow filtering dossiers/votes/scores/... by theme