Added 'add to playlist' functionality.
Added 'add to playlist' functionality. (forgot to add some files in last commit)
Merge branch 'master' of https://github.com/AJRenold/LearnAbout

* 'master' of https://github.com/AJRenold/LearnAbout:
  difficulty filters now working, added a file I forgot to add from earlier commit
  worked out bug in sidebar with duplicating categories
  routes changed from /category/ to /topic/, posts now have resource type and difficulty attributes
  sidebar topics and subtopics working pretty well
  sidebar working decently and allowing one topic filter. need to add more topic filters
  more work on sidebar, need to work on re-rendering sidebar after a category is clicked
  forgot to delete notification helper function
  removed notifications module from screen.scss
  sidebar working, starting to add categories