Use a different attachments folder per tenant

While this is not strictly necessary, it can help moving the data of one
tenant to a different server or removing it.

Note we're using subfolders inside the `tenants` subfolder. If we simply
used subfolders with the schema names, if the schema names were, for
instance, language codes like `es`, `en`, `it`, ... they would conflict
with the default subfolders used by Active Storage.

config/storage.yml

@@ -1,9 +1,9 @@
 local:
-  service: Disk
+  service: TenantDisk
   root: <%= Rails.root.join("storage") %>
 
 test:
-  service: Disk
+  service: TenantDisk
   root: <%= Rails.root.join("tmp/storage") %>
 
 # s3: