Rails 6 update notes

[viktorsmari]

Fablabs.io is now on Rails 5.2

Here are notes on how to upgrade to Rails 6:

• [ ] Change to rails 6 in Gemfile
• [ ] Disable gem rocket_pants in Gemfile or see if the gem supports Rails 6. See #577
• [ ] Perform bundle update rails i18n-tasks
• [ ] Run and fix all tests with bundle exec rspec
• [ ] Preferably also update ruby version to latest 2.7.x (remember to also update in Dockerfile)