Syntax fixes and better tests for helper methods. Updated docs.

CHANGELOG

app/controllers/admin/groups_controller.rb

app/controllers/groups_controller.rb

app/helpers/groups_helper.rb

app/views/admin/groups/_form.html.haml

app/views/groups/_group_lfs_settings.html.haml

app/views/groups/edit.html.haml

doc/api/groups.md

lib/api/entities.rb

spec/helpers/groups_helper_spec.rb

spec/models/group_spec.rb