chore(repo): set l10n drivers as owners of .ftl files

Because:

* localization team should be notified for changes to Fluent files, so that review can happen during development instead of after landing

This commit:

* update CODEOWNERS with l10n project owners

Co-authored-by: Francesco Lodolo <flod@lodolo.net>

.github/CODEOWNERS

@@ -1 +1,3 @@
 * @mozilla/fxa-devs
+
+*.ftl @mozilla/fxa-l10n