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/models/tag.rb

@@ -0,0 +1,22 @@
+class Tag < ActiveRecord::Base
+
+  has_many :exercise_tags
+  has_many :exercises, through: :exercise_tags
+
+  validates_uniqueness_of :name
+
+  def destroy
+    if (can_be_destroyed?)
+      super
+    end
+  end
+
+  def can_be_destroyed?
+    !exercises.any?
+  end
+
+  def to_s
+    name
+  end
+
+end