Upgrade to Rails 6.1

Note that `Capybara.app_host` now returns `nil` by default and that
breaks tests using `lvh.me` or our custom `app_host` method, so we're
setting `Capybara.app_host` to the value it had in earlier versions of
Rails. I also haven't found a way to remove the code to set the
integration session host in relationable tests which I mentioned in
commit ffc14e499.

Also note that we now filter more parameters, and that they match
regular expressions, so filtering `:passw` means we're filtering
`passwd`, `password`, ...

spec/rails_helper.rb

@@ -67,6 +67,7 @@ end
 Capybara.exact = true
 Capybara.enable_aria_label = true
 Capybara.disable_animation = true
+Capybara.app_host ||= "http://127.0.0.1"
 
 OmniAuth.config.test_mode = true