FEATURE: Optionally skip using full_name when suggesting usernames (#16592)

This commit introduces a new site setting: `use_name_for_username_suggestions` (default true)

Admins can disable it if they want to stop using Name values when generating usernames for users. This can be useful if you want to keep real names private-by-default or, when used in conjunction with the `use_email_for_username_and_name_suggestions` setting, you would prefer to use email-based username suggestions.

spec/models/discourse_connect_spec.rb

@@ -450,6 +450,20 @@ describe DiscourseConnect do
     expect(user.username).to eq sso.name
   end
 
+  it "stops using name as a source for username suggestions when disabled" do
+    SiteSetting.use_name_for_username_suggestions = false
+
+    sso = new_discourse_sso
+    sso.external_id = "100"
+
+    sso.username = nil
+    sso.name = "John Smith"
+    sso.email = "mail@mail.com"
+
+    user = sso.lookup_or_create_user(ip_address)
+    expect(user.username).to eq "user"
+  end
+
   it "doesn't use email as a source for username suggestions by default" do
     sso = new_discourse_sso
     sso.external_id = "100"