[ticket/12311] metadata mgr should validate license instead of licence

For consistency, I also switched licence to license where it is displayed
to the user in the extensions manager. We shouldn't cause any confusion
about the spelling. The composer schema wants it to be "[license](https://getcomposer.org/doc/04-schema.md#license)"

After this is merged wikis and an announcement in the Extension Development
forums should be made notifying extension developers to change their
composer.json files from licence to license too.

http://tracker.phpbb.com/browse/PHPBB3-12311

PHPBB3-12311