feat(coderd): add company logo when available for email notifications (#14935)

This PR aims to close #14253 

We keep the default behavior using the Coder logo if there's no logo
set.
Otherwise we want to use the logo based on the URL set in appearance.

---------

Co-authored-by: defelmnq <yvincent@coder.com>

coderd/notifications/dispatch/utils_test.go

@@ -0,0 +1,10 @@
+package dispatch_test
+
+func helpers() map[string]any {
+	return map[string]any{
+		"base_url":     func() string { return "http://test.com" },
+		"current_year": func() string { return "2024" },
+		"logo_url":     func() string { return "https://coder.com/coder-logo-horizontal.png" },
+		"app_name":     func() string { return "Coder" },
+	}
+}