Upgrade to Rails 6

Rails 6 has been released!

https://weblog.rubyonrails.org/2019/8/15/Rails-6-0-final-release/

There are known issues / deprecations that we have to fix first:

https://gitlab.com/groups/gitlab-org/-/issues?scope=all&utf8=%E2%9C%93&state=opened&label_name[]=rails6