Rework backpack (fixes #54)

Preparing the next releases, splitting stuff appart

* [x] Added setting to disable backpack
* [x] Removed fake tabs in trek detail panel top panel
* [x] Removed backpack from results panel, now button aside *Share*
* [x] Refresh popover on add/remove trek from backpack
* [x] Remove trek from backpack list
* [ ] Fix position of backpack button
* [ ] Fix apparence of popover for long names
* [ ] Find strategy when landing on detail page (general refactor to be made to manage proper collections)
* [ ] Fix flatpages without fake tabs