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

db/migrate/20170205163247_create_exercise_collections.rb

@@ -0,0 +1,14 @@
+class CreateExerciseCollections < ActiveRecord::Migration
+  def change
+    create_table :exercise_collections do |t|
+      t.string :name
+      t.timestamps
+    end
+
+    create_table :exercise_collections_exercises, id: false do |t|
+      t.belongs_to :exercise_collection, index: true
+      t.belongs_to :exercise, index: true
+    end
+
+  end
+end

db/migrate/20170205165450_create_proxy_exercises.rb

@@ -0,0 +1,23 @@
+class CreateProxyExercises < ActiveRecord::Migration
+  def change
+    create_table :proxy_exercises do |t|
+      t.string :title
+      t.string :description
+      t.string :token
+      t.timestamps
+    end
+
+    create_table :exercises_proxy_exercises, id: false do |t|
+      t.belongs_to :proxy_exercise, index: true
+      t.belongs_to :exercise, index: true
+      t.timestamps
+    end
+
+    create_table :user_proxy_exercise_exercises do |t|
+      t.belongs_to :user, polymorphic: true, index: true
+      t.belongs_to :proxy_exercise, index: true
+      t.belongs_to :exercise, index: true
+      t.timestamps
+    end
+  end
+end

db/migrate/20170205210357_create_interventions.rb

@@ -0,0 +1,16 @@
+class CreateInterventions < ActiveRecord::Migration
+  def change
+    create_table :user_exercise_interventions do |t|
+      t.belongs_to :user, polymorphic: true
+      t.belongs_to :exercise
+      t.belongs_to :intervention
+      t.timestamps
+    end
+
+    create_table :interventions do |t|
+      t.string :name
+      t.text :markup
+      t.timestamps
+    end
+  end
+end

db/migrate/20170206141210_add_tags.rb

@@ -0,0 +1,19 @@
+class AddTags < ActiveRecord::Migration
+
+  def change
+    add_column :exercises, :expected_worktime_seconds, :integer, default: 0
+    add_column :exercises, :expected_difficulty, :integer, default: 1
+
+    create_table :tags do |t|
+      t.string :name, null: false
+      t.timestamps
+    end
+
+    create_table :exercise_tags do |t|
+      t.belongs_to :exercise
+      t.belongs_to :tag
+      t.integer :factor, default: 0
+    end
+  end
+
+end

db/migrate/20170206152503_add_user_feedback.rb

@@ -0,0 +1,11 @@
+class AddUserFeedback < ActiveRecord::Migration
+  def change
+    create_table :user_exercise_feedbacks do |t|
+      t.belongs_to :exercise, null: false
+      t.belongs_to :user, polymorphic: true, null: false
+      t.integer :difficulty
+      t.integer :working_time_seconds
+      t.string :feedback_text
+    end
+  end
+end