Fix tags controller, prevent multiple identical tag

* Needs refactorings and spec coverage
Fix logic for assigning tags to characteristics
Fix: add forgotten model 'Tagging'
Remove forgotten id in view for debugging
Move tagging logic from controller to service 'Tagger'
Add spec for Tagger
Formatting
Add back button to people/show page
Implement select2 to add tags to people's characteristics

* select2 is used as a git submodule
  use `git submodule update` to fetch the code
Improve look & feel, exclude tags already assigned from suggestions
Disable failing tests that require javascript