Extended Exercises by worktime, difficulty and tags, added ProxyExercises as prework for recommendations

Tags can be added to exercises in the edit view. Tags can monitored under /tags.
Added the concept of ProxyExercises which are a collection of Exercises. They can be found under /proxy_exercises
Added Interventions as prework to show interventions later to the user.
Added exercise/[:id]/working_time to return the working time of the user in this exercise and the average working time of all users in this exercise

app/views/exercises/_form.html.slim

@@ -32,6 +32,25 @@
     label
       = f.check_box(:allow_auto_completion)
       = t('activerecord.attributes.exercise.allow_auto_completion')
+  .form-group
+    = f.label(t('activerecord.attributes.exercise.difficulty'))
+    = f.number_field :expected_difficulty, in: 1..10, step: 1
+  .form-group
+    = f.label(t('activerecord.attributes.exercise.worktime'))
+    = f.number_field "expected_worktime_minutes", value: @exercise.expected_worktime_seconds / 60, in: 1..1000, step: 1
+  h2 Tags
+  .table-responsive
+    table.table
+      thead
+        tr
+          th = t('activerecord.attributes.exercise.selection')
+          th = sort_link(@search, :title, t('activerecord.attributes.tag.name'))
+          th = t('activerecord.attributes.tag.difficulty')
+        = collection_check_boxes :exercise, :tag_ids, @exercise_tags, :tag_id, :id do |b|
+          tr
+            td = b.check_box
+            td = b.object.tag.name
+            td = number_field "tag_factors[#{b.object.tag.id}]", :factor, :value => b.object.factor, in: 1..10, step: 1
   h2 = t('activerecord.attributes.exercise.files')
   ul#files.list-unstyled.panel-group
     = f.fields_for :files do |files_form|