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/intervention.rb

@@ -0,0 +1,15 @@
+class Intervention < ActiveRecord::Base
+
+  NAME = %w(overallSlower longSession syntaxErrors videoNotWatched)
+
+  has_many :user_exercise_interventions
+  has_many :users, through: :user_exercise_interventions, source_type: "ExternalUser"
+  #belongs_to :user, polymorphic: true
+  #belongs_to :external_users, source: :user, source_type: ExternalUser
+  #belongs_to :internal_users, source: :user, source_type: InternalUser, through: :user_interventions
+ # alias_method :users, :external_users
+  #has_many :exercises, through: :user_interventions
+
+  validates :name, inclusion: {in: NAME}
+
+end