Update to PHPBB 3.3
This is an eventual third step in the PHPBB migration. 3.2 is still supported, but 3.3 is the latest version. 3.3. requires PHP 7, while the VM is currently on 5.6. Once the other sites on the VM have been updated (if needed) and the VM is running PHP 7, this can be deployed.
This is branched off of !2 (merged), so should not need rebasing once !2 (merged) is merged.