Add permission level to groups

app/controllers/explore/groups_controller.rb

app/controllers/groups_controller.rb

app/finders/groups_finder.rb

app/helpers/groups_helper.rb

app/helpers/visibility_level_helper.rb

app/models/ability.rb

app/models/group.rb

app/views/groups/edit.html.haml

app/views/groups/new.html.haml

app/views/shared/_group_tips.html.haml

db/migrate/20160301124843_add_visibility_level_to_groups.rb

db/schema.rb

spec/finders/groups_finder_spec.rb

spec/helpers/groups_helper.rb

spec/helpers/visibility_level_helper_spec.rb

spec/models/group_spec.rb