Allow rendering different views per tenant

Sometimes it might be convenient to use completely different views for
different tenants. For example, a certain tenant might use a footer that
has nothing to do with the default one.

For these cases, instead of adding `case Tenant.current_schema`
conditions to the view, it might be tidier to use a different file.

For this purpose, we're using Rails variants [1], which means that a
tenant named `mytenant` will use a template ending with
`.html+mytenant.erb` if it's available.

This works with components too, but has a limitation: when using the
`custom/` folder to add ERB files for a tenant, the default tenant ERB
file needs to be added to the `custom/` folder as well; if there aren't
changes to this file, a symbolic link will do.

For example, if we're writing a custom `admin/action_component` view for
the tenant `milky-way` but don't need to change this file for the
default tenant:

1. Create `app/components/custom/admin/action_component.rb` according to
   the components customizations documentation [2]
2. Create the custom view for the `milky-way` tenant and save it under
   `app/components/custom/admin/action_component.html+milky-way.erb`
3. Enter the `app/components/custom/admin/` folder and run `ln -s
   ../../admin/action_component.html.erb`

We're also adding some controller tests. Since Rails doesn't load the
middleware during controller tests, we're stubbing the `current_schema`
method directly instead of changing the subdomain of the request.

[1] https://guides.rubyonrails.org/v6.0/layouts_and_rendering.html#the-variants-option
[2] https://docs.consulproject.org/docs/english-documentation/customization/components

app/controllers/application_controller.rb

@@ -1,6 +1,7 @@
 require "application_responder"
 
 class ApplicationController < ActionController::Base
+  include TenantVariants
   include GlobalizeFallbacks
   include HasFilters
   include HasOrders