Install intl extension

Summary:
The intl extension provides functions for international domain names,
or a grapheme based alternative to mbstring.

It's used by MediaWiki, and at Nasqueron, by Notifications Center.

Ref T1813

Test Plan: php -i | grep ICU

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: Nasqueron Docker deployment squad

Tags: Docker images

Maniphest Tasks: T1813

Differential Revision: https://devcentral.nasqueron.org/D2958