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

config/routes.rb

@@ -60,12 +60,29 @@ Rails.application.routes.draw do
     member do
       post :clone
       get :implement
+      get :working_times
       get :statistics
       get :reload
       post :submit
     end
   end
 
+  resources :proxy_exercises do
+    member do
+      post :clone
+      get :reload
+      post :submit
+    end
+  end
+
+  resources :tags do
+    member do
+      post :clone
+      get :reload
+      post :submit
+    end
+  end
+
   resources :external_users, only: [:index, :show], concerns: :statistics do
     resources :exercises, concerns: :statistics
   end