Youtube Refactoring

PivotalTracker Story: https://www.pivotaltracker.com/story/show/69074352

* Refactored `Youtube` into `YoutubeVideos` and `YoutubeHelper`
* Plan to move all the API calls to background job to improve performance in a later pull request.

1. `app/controllers/projects_controller.rb` - Changed the way to youtube videos are fetched to use the new YoutubeVideos.
1. `app/controllers/users_controller.rb` - Changed the way to youtube videos are fetched to use the new YoutubeVideos
1. ` app/helpers/application_helper.rb ` - Removing unused and useless methods.
1. `app/helpers/users_helper.rb` - Everything related to YouTube moved to `YoutubeService` and `YoutubeHelper`.
1. `app/helpers/youtube_helper.rb` - Contains methods used to fetch a user's youtube related information when he syncs with youtube.
1. `app/models/project.rb` - Added a members instance method, to remove duplication throughout the app.
1. `lib/youtube_videos.rb` - This YoutubeService encapsulated all the functionality for fetching both user videos and project videos.
1. `features/project_pivotal_stories.feature` - Fixing the project pivotal tracker stories feature to stub out the request for youtube on the project index page.
1. `features/step_definitions/youtube_steps.rb`, `features/tabs.feature` - Switch to the new YoutubeService.
same here, be consistent with the rest of the tests and use `.and_return(...)`