Summary:
Shows a placeholder flag icon for the language picker button when a country flag
isn't available.
The country flag icon is displayed on top of the placeholder icon. For countries
where an icon isn't available, the placeholder will then become visible.
This fixes a bug where no icon was shown for languages that didn't have a flag
icon available.
Test Plan: Tested manually.
Reviewers: jarek
Reviewed By: jarek
Subscribers: jarek
Differential Revision: https://phab.getgrist.com/D3805
