Authenticate action import_proforma_xml via OAuth2

- currently done manually, we should definitely consider a gem here
  (doorkeeper)
- right now, the provided Bearer token from the Authorization header is
  just looked up in the configured code harbor links of the current user,
  if one is found -> authenticated -> exercise gets imported

(leoselig/codeocean#1)

app/controllers/exercises_controller.rb

@@ -67,10 +67,54 @@ class ExercisesController < ApplicationController
   end
 
   def import_thin_common_cartridge
-    logger.info(request.headers['Authorization'])
-    logger.info(request.headers['Authorisation'])
-    render :nothing => true, :status => 200, :content_type => 'text/html'
+    begin
+      user = user_for_oauth2_request()
+      exercise = Exercise.new
+      exercise.from_proforma_xml(request.body.read)
+      exercise.update(:user => user)
+      saved = exercise.save
+      if saved
+        render :text => 'SUCCESS', :status => 200
+      else
+        render :text => 'Invalid exercise', :status => 400
       end
+    rescue => error
+      if error.class == Hash
+        render :text => error.message, :status => error.status
+      else
+        raise error
+        render :text => '', :status => 500
+      end
+    end
+  end
+
+  def user_for_oauth2_request
+    authorizationHeader = request.headers['Authorization']
+    if authorizationHeader == nil
+      raise ({status: 401, message: 'No Authorization header'})
+    end
+
+    oauth2Token = authorizationHeader.split(' ')[1]
+    if oauth2Token == nil || oauth2Token.size == 0
+      raise ({status: 401, message: 'No token in Authorization header'})
+    end
+
+    user = user_by_code_harbor_token(oauth2Token)
+    if user == nil
+      raise ({status: 401, message: 'Unknown OAuth2 token'})
+    end
+
+    return user
+  end
+  private :user_for_oauth2_request
+
+  def user_by_code_harbor_token(oauth2Token)
+    link = CodeHarborLink.where(:oauth2token => oauth2Token)[0]
+    if link != nil
+      return link.user
+    end
+  end
+  private :user_by_code_harbor_token
 
   def exercise_params
     params[:exercise].permit(:description, :execution_environment_id, :file_id, :instructions, :public, :hide_file_tree, :team_id, :title, files_attributes: file_attributes).merge(user_id: current_user.id, user_type: current_user.class.name)