config: fix email assets not having the full URL domain prepended

When sending emails, the mailer doesn't have access to the request
host. It needs to infer it by itself.

For this we need two settings:

- action_mailer.default_url_options, to generate urls to routes
- action_mailer.asset_host, to generate full urls to assets

Only the first one of these settings was set in production.

Fix #2518

config/environments/development.rb

@@ -46,6 +46,13 @@ Rails.application.configure do
 
   # Action Mailer settings
   config.action_mailer.delivery_method = :letter_opener_web
+  # Configure default root URL for generating URLs to routes
+  config.action_mailer.default_url_options = {
+    host: 'localhost',
+    port: 3000
+  }
+  # Configure default root URL for email assets
+  config.action_mailer.asset_host = "http://" + ENV['APP_HOST']
 
   Rails.application.routes.default_url_options = {
     host: 'localhost',