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`, ...

config.ru

@@ -1,4 +1,6 @@
 # This file is used by Rack-based servers to start the application.
 
-require ::File.expand_path("../config/environment", __FILE__)
+require_relative "config/environment"
+
 run Rails.application
+Rails.application.load_server