Follow Zeitwerk conventions for file structure

Even though we don't load this file with Zeitwerk, we're doing it for
consistency.

If we tried to load this file using Zeitwerk, without this change we'd
get an error:

```
NameError: uninitialized constant OmniauthWordpress
```

config/initializers/devise.rb

@@ -1,4 +1,4 @@
-require Rails.root.join("lib", "omniauth_wordpress")
+require Rails.root.join("lib", "omni_auth", "strategies", "wordpress")
 
 # Use this hook to configure devise mailer, warden hooks and so forth.
 # Many of these configuration options can be set straight in your model.