Limit programming groups to two people

* adapt text on programming_groups/new site
* in German change from 'Sie' to 'du'

app/models/programming_group.rb

@@ -11,7 +11,8 @@ class ProgrammingGroup < ApplicationRecord
   has_many :runners, as: :contributor, dependent: :destroy
   belongs_to :exercise
 
-  validate :group_size
+  validate :min_group_size
+  validate :max_group_size
   validate :no_erroneous_users
   accepts_nested_attributes_for :programming_group_memberships
 
@@ -71,12 +72,18 @@ class ProgrammingGroup < ApplicationRecord
 
   private
 
-  def group_size
+  def min_group_size
     if users.size < 2
       errors.add(:base, :size_too_small)
     end
   end
 
+  def max_group_size
+    if users.size > 2
+      errors.add(:base, :size_too_large)
+    end
+  end
+
   def no_erroneous_users
     @erroneous_users.each do |partner_id|
       errors.add(:base, :invalid_partner_id, partner_id:)

app/views/programming_groups/_form.html.slim

@@ -1,7 +1,7 @@
 = form_for(@programming_group, url: exercise_programming_groups_path) do |f|
   = render('shared/form_errors', object: @programming_group)
   .mb-3
-    = f.label(:programming_partner_ids, class: 'form-label')
+    = f.label(:programming_partner_id, class: 'form-label')
     = f.text_field(:programming_partner_ids, class: 'form-control', required: true, value: (@programming_group.programming_partner_ids - [current_user.id_with_type]).join(', '))
-    .help-block.form-text = t('.hints.programming_partner_ids')
+    /.help-block.form-text = t('.hints.programming_partner_ids')
   .actions.mb-0 = render('shared/submit_button', f: f, object: @programming_group)

app/views/programming_groups/new.html.slim

@@ -1,4 +1,4 @@
-h1 = t('shared.new_model', model: ProgrammingGroup.model_name.human)
+h1 = t('programming_groups.new.create_programming_pair')
 .row
   .col-md-6
     p