remove public field from namespace and refactoring

app/controllers/explore/groups_controller.rb

app/controllers/users_controller.rb

app/finders/groups_finder.rb

app/finders/joined_groups_finder.rb

app/helpers/search_helper.rb

app/models/ability.rb

app/models/group.rb

app/views/groups/edit.html.haml

app/views/groups/show.html.haml

db/migrate/20151228111122_remove_public_from_namespace.rb

db/schema.rb

features/explore/groups.feature

spec/finders/groups_finder_spec.rb

spec/finders/joined_groups_finder_spec.rb

spec/helpers/search_helper_spec.rb

spec/models/group_spec.rb