Replace manual activerecord translations with helpers

app/views/study_groups/_form.html.slim

@@ -4,12 +4,12 @@
     = f.label(:name, class: 'form-label')
     = f.text_field(:name, class: 'form-control', required: true)
 
-  h3 = t('activerecord.attributes.study_group.members')
+  h3 = StudyGroup.human_attribute_name('members')
   .table-responsive
     table.table class=(@members.present? ? 'sortable' : '')
       thead
         tr
-          th = t('activerecord.attributes.exercise.selection')
+          th = Exercise.human_attribute_name('selection')
           th = t('navigation.sections.users')
       = collection_check_boxes :study_group, :study_group_membership_ids, @members, :id, :id do |b|
         tr