Make it possible to disable tenants

Note we could use `acts_as_paranoid` with the `without_default_scope`
option, but we aren't doing so because it isn't possible to consider
deleted records in uniqueness validations with the paranoia gem [1].
I've added tests for these cases so we don't accidentally add
`acts_as_paranoid` in the future.

Also note we're extracting a `RowComponent` because, when
enabling/disabling a tenant, we're also enabling/disabling the link
pointing to its URL, and so we need to update the URL column after the
AJAX call.

[1] See issues 285 and 319 in https://github.com/rubysherpas/paranoia/

config/routes/admin.rb

@@ -284,7 +284,12 @@ namespace :admin do
     delete :cancel, on: :collection
   end
 
-  resources :tenants, except: [:show, :destroy]
+  resources :tenants, except: [:show, :destroy] do
+    member do
+      put :hide
+      put :restore
+    end
+  end
 end
 
 resolve "Milestone" do |milestone|