Added Jinja2 Autoescape to playlists_view.html

Found that it was possible to inject arbitrary javascript in the onClick attribute. To prevent this, I have used the autoescape functionality of Jinja2, which **should** fix the issue. I have not yet tested if it does though.

In case you were wondering, JavaScript could be added to song.name and the song.artist.
Merge pull request #1 from infosec-au/patch-1

Added Jinja2 Autoescape to playlists_view.html