Upgrade from Rails 5.2.2 to Rails 6.0.0

[alaxalves]

Follow the steps below will definitely help:
https://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-5-2-to-rails-6-0
This depends on #602

[jywarren]

Hi! Do we have a good plan for merging a PR that contains only what we can already merge under Rails 5 in prep, that will be required for Rails 6? And a place where we're discussing any needed planning or ideas for the Debian Rails schedule? Thank you!

[alaxalves]

Discussions and merge available to #824