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/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