Link to the slack channel in the contribution guidlines appears broken

[RichardJActon]

Hi just wanted to let you know that the link to the slack channel in contributing links to a heroku page with 'there is nothing here yet': https://openmooc-ers-slackin.herokuapp.com/ and not to a slack channel. I had the same issue with the link on the about page on the website: https://opensciencemooc.eu/about/.