Main navigation refactor

Goal : introduce genericity in JS, CSS, results and detail pages.

**This will be the base branch of Geotrek-rando 2.0**

* [x] Move reusable parts to core
* [x] Split reusable parts of sync to core
* [x] Get rid of propertiers (ref #118)
* [x] Introduce POIs
* [x] POI detail page (rename trekking/detail_panel to trek_detail)
* [x] Fix POI list
* [x] park centered
* [x] Move attachments block to core/detail_panel
* [x] Move disqus to core/detail_panel
* [x] Write minimal test for POI panel (+setting OPENED)
* [x] Review history of detail_panel.html and backport
* [x] Review complete changelog since 1.32 to check if any regression