Add username template variables to Welcome User email template (#20352)

* Add username and name_or_username variables to SystemMessage defaults

* Allow username and name variables on welcome_user email template overrides

* Satisfy linting

* Add test

app/models/translation_override.rb

@@ -35,6 +35,7 @@ class TranslationOverride < ActiveRecord::Base
       optional_cat
       optional_tags
     ],
+    %w[system_messages.welcome_user] => %w[username name name_or_username],
   }
 
   include HasSanitizableFields

lib/system_message.rb

@@ -88,6 +88,8 @@ class SystemMessage
     {
       site_name: SiteSetting.title,
       username: @recipient.username,
+      name: @recipient.name,
+      name_or_username: @recipient.name.presence || @recipient.username,
       user_preferences_url: "#{@recipient.full_url}/preferences",
       new_user_tips:
         I18n.with_locale(@recipient.effective_locale) do

spec/models/translation_override_spec.rb

@@ -84,6 +84,19 @@ RSpec.describe TranslationOverride do
         end
       end
 
+      describe "with valid custom interpolation keys" do
+        it "works" do
+          translation_override =
+            TranslationOverride.upsert!(
+              I18n.locale,
+              "system_messages.welcome_user.text_body_template",
+              "Hello %{name} %{username} %{name_or_username} and welcome to %{site_name}!",
+            )
+
+          expect(translation_override.errors).to be_empty
+        end
+      end
+
       describe "pluralized keys" do
         describe "valid keys" do
           it "converts zero to other" do