Improve news fetching performance by 25%
Improve ActiveRecord performance on homepage by 500% using includes
Optimise algorithm to find poles

* Implements Tag#poles
* Adds a big performance improvement
Optimise algorithm to find free tags

* Implements Tag#free
* Adds a big performance improvement
Sort on db to preserve ActiveRecord relation object
Replace 'find' by 'where' statements to preserve ActiveRecord relation
Remove unnecessary 'joins' statement
Add slight performance improvement in tag trees with eager loading
Fix failing spec receiving_a_digest_email_spec
Fix failing spec tags_presenter_spec
Disable creating an Activity when receiving a news digest
Rename spec changing_user_configuration to managing_user_configuration
Better implement accessing other's news configurations with feature spec