Move all mixins to geotrek.common
Move notion of publication to geotrek.common
Rename TREK_PUBLISHED_BY_LANG to PUBLISHED_BY_LANG
Generalize settings regarding completeness and image size
Extract notion of publication
Make POIs publishable
Share pieces of detail templates for publishable objects
Filter POI by published status in trek publication views
Minor changes
Fix migration for poi published to true in default language
Merge pull request #1179 from makinacorpus/refactor_publication_models_views

Generalize notions of publication