update method also updates the youtube urls (further room for improvements ;))
refactors thumbnail url getter
adds a fallback icon for the back arrow button
with bugfixes:
Merge branch 'master' of heroku.com:iupb-beta into develop
# By Dirk Schumacher
# Via Dirk Schumacher
* 'master' of heroku.com:iupb-beta:
  update method also updates the youtube urls (further room for improvements ;))
fixes and closes #27