Test environment stability improvements (#703)

* Add climate_control gem and test helper

* Replace ENV mods in upgrades test

* Replace ENV mods in registrations test

* Remove ENV references in hostings controller

* Update ENV refs in mailer test

* ActiveStorage cleanup

* Consolidate queue config so appropriate adapter runs in test environment

* Make test environment more explicit

* Centralize self hosting config

* Remove flaky system test

app/controllers/concerns/self_hostable.rb

@@ -7,6 +7,6 @@ module SelfHostable
 
   private
     def self_hosted?
-      ENV["SELF_HOSTING_ENABLED"] == "true"
+      Rails.configuration.app_mode.self_hosted?
     end
 end